Brake Types

Drum brake system

Summary

The primary components of the drum brake system are: the backing plate, the brake drum & brake shoe assembly, the wheel cylinder, retaining clips & springs and automatic brake self-adjuster.

Drum brakes are still found on older vehicles, and on cars with a combination of both disc and drum brakes.

The main components of the drum brake system are:

Backing plate

Brake drum & brake shoe assembly

Wheel cylinder

Retaining clips & springs

Automatic brake self-adjuster

Backing plate

The backing plate is made from steel and is attached to the steering or suspension components by bolts. It then supports the wheel cylinder, brake shoes and levers.

Brake drum & brake shoe assembly

The brake drum fits over the brake linings and forms the braking surface for these linings.

The brake shoe assembly consists of the steel shoe and the brake lining material.

Wheel cylinder

The wheel cylinder is attached to the backing plate. The pistons push against the brake shoes, which then make contact with the brake drum to slow or stop the vehicle.

Retaining clips & springs

The brakes are held against the backing plate by retaining clips and springs.

The hold down spring is used to retain the the brake shoe in position in relation to the backing plate. During vehicle operation it keeps the brake shoe in position.

Automatic brake self-adjuster

To manually adjust the brakes it may be necessary to release the adjusting lever away from the star wheel. To do this, insert a small screwdriver through the adjust slot in the backing plate and push on the adjusting lever. A brake adjuster can now be used to adjust the brakes in the usual way.

Drum brake operation

Summary

The drum brake system operates by forcing the friction-lined brake shoes against the inner surfaces of the rotating drums. The shoes are designed to operate with a self-energizing action.

Drum brakes were once common on all wheels of light vehicles, but now they are less commonly used, and even then usually only found on just the rear wheels, in disc-drum combinations.

The drum brake has two brake shoes with a friction material called a lining attached. These shoes expand against the inside surface of a brake drum, and slow the wheel down. The harder the linings are forced against the brake drum, the higher the braking force that is applied. They can be expanded mechanically, or hydraulically.

The main advantage claimed for drum brakes is that the shoe mountings can be designed to assist their own operation. This is called self-energizing. Less hydraulic pressure is then needed to stop the vehicle, which is why many older drum-braked vehicles didn’t use a brake booster.

The main disadvantage of drum brakes is that the friction area is almost entirely covered by lining, so most heat must be conducted through the drum to reach the outside air to cool. With hard use, this can cause overheating, and eventually brake 'fade'. Brake fade is the gradual loss of brake stopping power during prolonged or strenuous use. Very high temperatures occur at the brake drum, and that causes deterioration in the frictional value of the lining or pad material. This is common in drum brakes.

Another problem with drum brakes is that it is difficult to get water out of the drum. If a vehicle is driven through water, it takes longer to get the brakes working effectively again.

Three brake designs are in general use:

single leading shoe,

twin leading shoe; and

duo-servo.

Each one uses the wedging or self-energizing action of the brake shoe, to assist the lining to grip the rotating drum when the brakes are applied. The twin-leading shoe has an actuator for each brake shoe. The actuator can be mechanical, however a hydraulic actuator is popular on light vehicles. The hydraulic actuator is called the wheel cylinder.

Some brakes have two wheel cylinders, with one piston in each cylinder. When the brakes are applied, hydraulic pressure forces each piston to move outwards, pushing on one end of the brake shoe. The direction of rotation of the drum produces a wedging action on both brake shoes, so they are both called leading shoes.

This system was once popular on front wheels because it is very efficient in the forward direction. This is due to the self-energizing or self-wedging action of the shoes as the drum rotates. Its main disadvantage is that it is only about 30% as efficient in reverse, so it is usually combined with a single leading shoe arrangement on the rear to provide a balanced system.

The single leading shoe system uses a single wheel cylinder with two pistons. When the brakes are applied, both shoes press against the brake drum. One shoe is called leading shoe, the other is called trailing. The leading shoe tends to be self-energized, while the trailing shoe tends to be forced off the drum.

This arrangement is common on rear wheels as they work equally well in forward and reverse, so it makes an effective handbrake. They can also have a self-adjusting mechanism.

The duo-servo design also uses one wheel cylinder with two pistons. It is a high energy brake, that is, it exerts large self-energizing forces. The lower ends of the shoes are linked but aren’t firmly anchored to the backing plate. This lets the complete shoe assembly float, within limits.

When the brakes are applied, both shoes are carried around by the drum, until the secondary shoe contacts the anchor pin. The self-energizing force of the primary shoe and its wheel cylinder application force is now transferred to the secondary shoe through the lower linkage. Force is then being applied to the secondary shoe from both ends – the wheel cylinder at the top, and the linkage from the primary shoe at the bottom. The primary shoe has the shorter lining and is always fitted ahead of the wheel cylinder in terms of drum rotation. It’s most important that the shoes are fitted correctly, since it’s the secondary shoe that does most of the work. The linings may also have different frictional values. The colors of the retraction springs indicate different spring strengths. This design is common on rear wheels and it works well in both directions.

Drum brake systems need to be adjusted to allow for wear of the lining. If they are not adjusted, pedal travel will be too long to be safe.

Brake linings & shoes

Summary

The brake lining is a specialized friction material, riveted or bonded to a steel shoe. The shoe transfers the wheel cylinder movement and forces the lining against the rotating drum.

The drum brake uses brake shoes that have friction material called linings attached to them. Linings can be riveted or more often bonded to the brake shoes. Most linings have a manufacturer's code on the edge of the lining which will identify their specified co-efficient of friction.

The composition of the friction material affects brake operation. Linings which provide good braking with low pedal pressures tend to lose efficiency when they get hot. This means the stopping distance will be increased. Linings which maintain a stable friction co-efficient over a wide temperature range, generally require higher pedal pressures to provide efficient braking, and may need the use of a booster.

This friction material was once made of asbestos but concerns about health problems associated with asbestos have led to the use of non-asbestos alternatives. However, if you are removing older brake linings it is impossible to tell whether asbestos was used as a component in their manufacture, so they must be handled with care. Wear protective clothing and make sure you do not inhale any of the brake dust residue, and very importantly, never use an air hose to blow brake off any component.

Backing plate

Summary

The backing plate is bolted to the axle housing or suspension member. It provides a mounting for the wheel cylinders and brake shoes to act against the rotating drum.

All of the brake unit components, except the brake drum, are mounted on a backing plate bolted to the vehicle axle housing or suspension.

The backing plate is usually pressed from heavy gauge steel. It has a raised outer edge that fits into a groove or recess in the brake drum and helps keep out any dust or dirt.

Some vehicles have manual brake adjusters so openings are usually provided to allow for adjustments without having to remove the wheel and brake drum.

Wheel cylinders

Summary

The wheel cylinder and piston react to hydraulic pressure from the master cylinder, and the outward movement of the piston forces the shoe and lining against the drum.

The wheel cylinder is located inside the brake drum, and bolted to the backing plate. It converts hydraulic pressure from the master cylinder into mechanical force that pushes the brake linings against the brake drum.Wheel cylinders are either:

Single piston/single action; and

Dual action/double cylinder with a piston at each end.

They are usually made of cast iron or aluminum alloy, and they operate under difficult conditions of extreme pressures and temperatures.. Some are sleeved with stainless steel to be longer-wearing and more resistant to corrosion. Contamination, particularly from water, lowers the boiling point of the brake fluid and may cause pitting and fluid loss. The wheel cylinder cups seal the cylinder against fluid loss.

Wheel cylinders may be fitted with a spreader and a light expansion spring to keep the lips in contact with the cylinder during retraction and while at rest. This helps keep air out of the system. Most wheel cylinders are fitted with bleed nipples to allow air to be bled from the system after assembly, and a flexible cover, or boot, allows for piston movement and also keeps out dust and moisture.

Disc brake system

Summary

The primary components of disc brakes are: the rotor, caliper and brake pads.

Modern passenger vehicles are usually equipped with disc brakes on at least two wheels.

The primary components of the disc brakes are:

Rotor

Caliper

Brake pads

The rotor is the main rotating part of this brake system. It is hard wearing and resists the high temperatures that occur during braking. Its manufacturer will specify the minimum thickness for the

rotor. Rotors can be of a solid construction or slotted. The slotted rotor is referred to as a "ventilated disc".

Rotor

Disc brakes have rotors to dissipate heat so the brakes work efficiently. In high performance vehicles, the rotors are made from composite materials, ceramics, or carbon fiber; otherwise, they're made of iron.

Some of these rotors have directional vanes, which means the disc can only be fitted to one side of the vehicle.

Caliper

The caliper straddles the rotor, and houses the disc brake pads and activating pistons. The caliper is usually bolted to the steering knuckle or, in the case of a non-steer axle, to a suspension component.

Brake pads

The disc pads are located inside the caliper. The pads clamp onto the rotor to slow or stop the vehicle. The disc pad consists of a friction material bonded to a steel backing plate.

Disc brake operation

Summary

The disc brake system converts the hydraulic pressure generated at the master cylinder into a frictional clamping force against the rotating discs.

Disc brakes can be used on all four wheels of a vehicle, or combined with disc brakes on the front wheels and drum brakes on the rear.When the brake pedal is depressed, a push rod transfers the force through a brake booster to a hydraulic master cylinder. The master cylinder converts the force into hydraulic pressure, which is then transmitted via connecting pipes and hoses to one or more pistons at each brake caliper. The pistons operate on friction pads to provide a clamping force on a rotating flat disc that is attached to the wheel hub. This clamping tries to stop the rotation of the disc, and the wheel.

On non-driving wheels, the center of the brake disc or hub contains the wheel bearings. The hub can be part of the brake disc or a separate assembly between the wheel and hub with nuts or bolts. On driving wheels, the disc is mounted onto the driving axle and may be held in place by the wheel. On front wheel drive vehicles, it can be mounted on the front hub and wheel bearing assembly.

The brake caliper assembly is bolted to the vehicle axle housing or suspension. In most cases the brake is positioned as close as possible to the wheel, but there are exceptions. Some high-performance cars use inboard disc brakes on its rear wheels. The makers claim improved vehicle handling for this design because it reduces unsprung weight.

Applying brakes can absorb a lot of vehicle energy so friction between braking surfaces generates great heat. Brake parts withstand very high temperatures. Most of the friction area of a disc is exposed to air so cooling is far more rapid than for a drum brake. Unlike with drum brakes, brake fade is rare. Because of their shape, discs tend to throw off water. So after being driven through water, they operate almost immediately.

Disc brakes need much higher pressures to operate than drum brakes, so almost all disc brake systems need a power brake booster to help reduce the pedal forces that are needed from the

driver. Because of the high forces needed to apply a disc brake, using it as a handbrake is less common. Some vehicles build a drum brake into the center of the rear disc to provide for park brake operation.

Disc brake rotors

Summary

The brake disc rotates with the road wheel. It provides a smooth surface against which to force the brake pads, to slow or stop the vehicle.

The brake disc or rotor is the main rotating component of the disc brake unit.

It’s usually made of cast iron because it’s hard-wearing and can resist high temperatures.

On motorcycles, it is often made of stainless steel.

Most brake discs are stamped with the manufacturer’s minimum thickness specification. When the pad wears, if the thickness of the disc were below this minimum, the piston may go beyond the sealing edge.

Ventilated discs can be used to improve cooling. These slots are designed to use centrifugal force to cause airflow when the disc is rotating.

Some discs are drilled or slotted on their friction surface to improve cooling and assist with removing water.

Disc brake pads

Summary

A disc brake pad has a rigid, moulded, friction material bonded to a steel backing plate for support during brake application. It transforms the hydraulic force of the caliper into a frictional force against the disc.

Disc brake pads consist of friction material bonded onto a steel backing plate. The backing plate has lugs that locate the pad in the correct position in relation to the disc.

Calipers are usually designed so that the condition of the pads can be checked easily once the wheel has been removed, and to allow the pads to be replaced with a minimum of disassembly.

Some pads have a groove cut into the friction surface. The depth of this groove is set so that when it can no longer be seen, the pad should be replaced.

Some pads have a wire in the friction material at the minimum wear thickness. When the pad wears to this minimum thickness, the wire touches the disc as the brakes are applied. A warning light then tells the driver the disc pads are due for replacement.

The composition of the friction material affects brake operation. Materials which provide good braking with low pedal pressures tend to lose efficiency when they get hot. This means the stopping distance will be increased. Materials which maintain a stable friction co-efficient over a wide temperature range, generally require higher pedal pressures to provide efficient braking.

Disc rotors with holes or slots in them dissipate their heat faster, and also help to remove water from the surface of the pad in wet driving conditions. They also help to prevent the surface of the pad from becoming hard and glassy smooth from the friction and heat of use. However, this scraping action reduces the overall life of the brake pad, so these types of discs are generally only used in high performance or racing cars.

Disc brake calipers

Summary

Disc brake calipers provide a housing for the hydraulic piston or pistons that force the brake pads into contact with the disc.

The disc brake caliper assembly is bolted to the vehicle axle housing or suspension.

There are 2 main types:

fixed, and

sliding.

Fixed calipers can have 2, 3, or 4 pistons. 2-piston calipers have one piston on each side of the disc. Each piston has its own disc pad.

When the brakes are applied, hydraulic pressure forces both pistons inwards, causing the pads to come in contact with the rotating disc.

The sliding or floating caliper has 2 pads but only 1 piston. The caliper is mounted on pins or bushes that let it move from side to side.

When the brakes are applied, hydraulic pressure forces the piston inwards. This pushes the pad against the disc. The caliper is free to move on slides, so there is a clamping effect between the inner and outer pads. Equal force is then applied to both pads which clamp against the disc.

In disc brake calipers, the piston moves against a stationary square section sealing ring.

When the brakes are applied, the piston slightly deforms the seal.

When the brakes are released, the seal returns to its original shape. The action of this sealing ring retracts the piston to provide a small running clearance between the disc and pads. It also makes the brake self-adjusting.

Proportioning valves

Summary

The proportioning valve divides up the braking effort applied to front and rear wheels under heavy braking, according to how load is distributed across a vehicle.

The proportioning valve divides up the braking effort applied to front and rear wheels under heavy braking, according to how load is distributed across a vehicle.

The effectiveness of braking force is determined by tire-to-road friction. And this increases as load increases.

Applying the brakes causes the front of this vehicle to dip. This causes greater tire-to-road friction on the front tires, and less on the rear. This kind of change of load is called load transfer.

So, if equal braking force is applied to the front and rear wheels, the smaller rear load can make the rear wheels lock, and perhaps skid.

The braking force applied to the wheels needs to be adjusted to allow for changes in load.

Proportioning valve operation

Summary

The proportioning valve adjusts the braking force applied to the wheels to allow for changes in load.

The proportioning valve adjusts braking force to allow for load transfer. It can be pressure-sensitive, or load-sensitive.

The pressure-sensitive valve can be in the master cylinder, or in a separate unit in the rear brake circuit.

The load-sensitive type can be in the body or the axle, where it can respond to load changes, and change the braking effort as needed.

Master cylinder applications usually combine the proportioning valve with a pressure differential switch.

In normal braking, the poppet piston is held in a relaxed position by a large pressure spring. The poppet valve is held against its retainer by a light return spring, and fluid passes freely through the valve to the rear brakes.

In heavy braking, master cylinder pressure can reach a valve’s crack-point. The pressure applied to the 2 different areas of the poppet piston creates unequal forces. That moves the poppet piston against the large pressure spring. This action holds the conical section of the valve against the seat, which limits the pressure increase to the rear brakes.

As greater pedal force increases pressure in the master cylinder, fluid pressure rises on the smaller end of the piston. This combines with the force of the pressure spring to overcome the lower pressure now on the larger end. This forces the piston back, clear of the poppet valve.

The increased pressure now acts on the larger end of the poppet piston and again forces the piston forward to contact the valve.

When the pedal is released, the pressure of the rear brake fluid unseats the poppet valve, letting fluid return to the master cylinder. The pressure spring now returns the poppet piston to its relaxed position.

Should the front brake system fail, the warning lamp spool moves forward, taking the poppet valve with it. Pressure in the rear brakes rises and the piston moves forward but it can’t seal on the valve.

Should the rear brake system fail, the warning lamp spool will move backwards to activate the warning light. The proportioning valve doesn’t operate in this situation.

On a diagonally-divided system, the pressure-sensitive proportioning valve is usually located away from the master cylinder. There is one for each circuit. They each operate in a similar way to the pressure-sensitive proportioning valve located in the master cylinder, but without the pressure-differential warning light circuit.

The load-sensing proportioning valve is usually located in the rear brake circuit, on the chassis. A diagonally-split system may have 2 load-sensing proportioning valves, one for each brake. The unit is mounted on the chassis, around the rear suspension.

Disc brake system

Summary

The primary components of disc brakes are: the rotor, caliper and brake pads.

Modern passenger vehicles are usually equipped with disc brakes on at least two wheels.

The primary components of the disc brakes are:

Rotor

Caliper

Brake pads

The rotor is the main rotating part of this brake system. It is hard wearing and resists the high temperatures that occur during braking. Its manufacturer will specify the minimum thickness for the rotor. Rotors can be of a solid construction or slotted. The slotted rotor is referred to as a "ventilated disc".

Rotor

Disc brakes have rotors to dissipate heat so the brakes work efficiently. In high performance vehicles, the rotors are made from composite materials, ceramics, or carbon fiber; otherwise, they're made of iron.

Some of these rotors have directional vanes, which means the disc can only be fitted to one side of the vehicle.

Caliper

The caliper straddles the rotor, and houses the disc brake pads and activating pistons. The caliper is usually bolted to the steering knuckle or, in the case of a non-steer axle, to a suspension component.

Brake pads

The disc pads are located inside the caliper. The pads clamp onto the rotor to slow or stop the vehicle. The disc pad consists of a friction material bonded to a steel backing plate.

Brake friction materials

Summary

Brake pads and linings are made from materials which have a high coefficient of friction. The choice of material depends on the braking application, but it needs to be able to absorb and disperse large amounts of heat without braking performance being adversely affected.

Friction is the force that acts to prevent two surfaces in contact from sliding against each other. When friction occurs, the kinetic energy (motion) of the sliding surfaces is converted into thermal energy (heat).

Some combinations of materials, such as ice on glass, have a very low coefficient of friction. That means that there is very little friction between them, and therefore almost no sliding resistance. Rubber tires against a hard road surface have a high coefficient of friction, which means they tend to grip and resist sliding against each other.

Disc brake pads and drum brake linings are made from materials which have a high coefficient of friction. They also need to have an ability to absorb and disperse large amounts of heat without their braking performance being adversely affected.

As the heat in brake pads and linings builds up, the friction capability of the material – and consequently its stopping power – reduces. This is called brake fade, and minimizing or overcoming fade is a major factor in the design of brakes and the development of brake friction materials.

Brake friction materials were commonly made from asbestos compounds, because of the excellent heat resistance of that material, but as that has now been proven to be highly toxic, asbestos is generally banned and no longer used.

Brakes are now manufactured from a variety of different materials that may be:

non-asbestos organic

low metallic

semi-metallic

ceramic

The choice of compound depends on the application. Lighter passenger vehicles generate less heat in the brakes than heavy or high performance vehicles. The optimum brake composition for any given vehicle or use will therefore be a combination of weighted attributes which include:

stopping power

heat absorption and dispersion

resistance to fade

recovery speed from fade

wear rate

performance when wet

noise in operation

For instance, owners of small domestic vehicles will probably tend to value a longer pad life rather than higher performance in extreme conditions. In racing cars, however, fade resistance and stopping power at high speeds would be considered more important than noise levels or wear rate.

ABS brake system

Summary

The primary components of the antilock braking system are: The electronic control unit, hydraulic control unit or modulator, power booster & master cylinder assembly and wheel sensor unit.

ABS brakes

The antilock braking system is designed to prevent wheels locking or skidding, no matter how hard brakes are applied, or how slippery the road surface.

The primary components of the ABS braking system are:

Electronic control unit (ECU)

Hydraulic control unit or modulator

Power booster & master cylinder assembly

Wheel sensor unit

Electronic control unit (ECU)

The ECU is located inside the vehicle. It receives signals from the sensors in the circuit and controls the brake pressure at the road wheels according to the data analyzed by the Unit.

Hydraulic control unit or modulator

The location of the Hydraulic Control Unit, or Modulator, varies from manufacturer to manufacturer. Some locate it under the fender or hood.

It receives operating signals from the ECU to apply or release the brakes under ABS conditions.

Power booster & master cylinder assembly

The power booster and master cylinder assembly is mounted on the firewall and is activated when the driver pushes down on the brake pedal. It provides the power assistance required during braking.

Wheel sensor unit

The wheel sensor unit consists of a tooth rotor that rotates with the road wheels and a pick-up that is located in the wheel hub.

Antilock braking system operation

Summary

The antilock braking system prevents wheels locking or skidding, no matter how hard brakes are applied, or how slippery the road surface. Steering stays under control and stopping distances are generally reduced.

Applying brakes too hard, or on a slippery surface, can cause the wheels to lock. When wheels lock, steering control is lost and, in most cases, it produces longer stopping distances. The antilock braking system prevents wheels locking or skidding, no matter how hard brakes are applied, or how slippery the road surface. Steering stays under control and stopping distances are generally reduced.It consists of a brake pedal, a master cylinder, wheel speed sensors, the electronic control unit or ECU, and the hydraulic control unit, also called a hydraulic modulator.

The wheel speed sensor consists of a notched or toothed rotor that rotates with each wheel, and a pickup. As the wheel turns, a small voltage pulse is induced into the pickup and sent to the electronic control unit. When the brakes are applied, the wheel's speed of rotation changes. This sends a new signal to the ECU.

If the control unit detects that a wheel might lock, it sends a signal to the hydraulic control unit. In a three-channel system, the hydraulic control unit uses three solenoid valves to control brake pressure and prevent them locking.

The valves are in series with the brake master cylinder and the brake circuits. One operates for each of the front wheels and one controls both rear wheels. At the start of a journey, the ABS automatically checks itself. Any failure in the system lights up a warning light in the dash-panel.

Principles of ABS braking

Summary

The antilock braking system controls braking force by controlling the hydraulic pressure of the braking system, so that the wheels do not lock during braking.

Braking force and the tendency of the wheels to lock up are affected by a combination of factors such as the friction coefficient of the road surface, and the difference between the vehicle speed and the road wheel speed. The ABS prevents the road wheels from locking up during heavy braking by controlling the vehicle's brake system hydraulic pressure.

During normal braking, as the rotational speed of the wheel falls, no electric current flows from the ECU to the hydraulic unit. The solenoid valve is not energized. The brake master cylinder hydraulic pressure is applied to the brake unit, and the ABS is not involved. However, even though the ABS is passive during normal braking, its control module is constantly monitoring for rapid deceleration of any of the wheels.

If a wheel-speed sensor signals severe wheel deceleration - which means the wheel is likely to lock up - the ECU sends a current to the hydraulic unit. This energizes the solenoid valve. The action of the valve isolates the brake circuit from the master cylinder. This stops the braking pressure at that wheel from rising, and keeps it constant.

If the sensors signal the wheel is still decelerating too rapidly, the ECU sends a larger current to the hydraulic unit. The armature moves even further and opens the valve. It opens a passage from the brake circuit. Brake fluid is sent from the brake circuit back to the master cylinder. Pressure in the brake caliper circuit is reduced so that the wheel is braked less heavily.

If the wheel sensors indicate that lowering the brake pressure is letting the wheel accelerate again, the ECU stops sending current to the hydraulic unit and de-energizes the solenoid valve. This lets the pressure increase, so that the wheel is again decelerated.

This cycle repeats itself about four to six times per second.

It is normal in an ABS for the valves in the hydraulic control unit to keep changing position as they change the brake pressure that’s being applied. These changes in position may cause rapid pulsations to be felt through the brake pedal.

ABS master cylinder

Summary

The tandem master cylinder transforms applied brake force into hydraulic pressure which is transferred to the wheel units through two separate circuits. This provides residual braking in the event of fluid loss.

The master cylinder is connected to the brake pedal via a pushrod.

The ABS master cylinder is similar to the tandem master cylinder used in divided systems. It has a primary piston, and a secondary piston.

The secondary piston incorporates a centre valve. This controls the opening and closing of a supply port drilling in the piston. At rest, the supply port is open and connects the reservoir with the front brake circuits. The primary piston still has an inlet port, and a compensating port.

When the brake is applied, the primary piston moves, which closes its compensating port. Fluid pressure in the primary circuit rises. It acts with the primary piston spring, to move the secondary piston forward, closing the centre valve. Pressure builds in the secondary circuit. Pressure keeps building in both circuits, and applies the brakes in both circuits.

If the secondary circuit fails, the secondary piston is forced to the end of the cylinder. When it reaches the end, pressure builds in the primary circuit.

If the primary circuit fails, the primary piston contacts the secondary piston, and pushes it to operate the secondary circuit.

In normal operation when the pedal is released, the springs in the master cylinder push the pistons back more quickly than the fluid can flow back from the wheel brake units. This creates a low pressure area in front of each piston. Such low-pressures can cause air to be drawn into the system.

To prevent this, there are recuperating grooves in the primary piston and the seal. Fluid at atmospheric pressure flows through the inlet port, and past these grooves. When the primary piston is returned fully, any extra fluid coming back from the brake units displaces fluid into the reservoir, through the compensating port.

In the secondary circuit, fluid also at atmospheric pressure is forced back into the inlet port. The inlet port connects with the supply port drilling in the piston. Any difference in pressure lifts the centre valve from its seat, and lets fluid enter the chamber ahead of the secondary seal, and prevents low pressures developing.

When the piston has returned to the “rest” position, the seal is pulled off its seat by the action of the link and spring. This lets fluid still returning from the wheel units displace fluid back to the reservoir.

If braking conditions are such that the hydraulic modulator must return brake fluid to the master cylinder, then, for the front brake circuits, fluid is returned to the front section. This forces the secondary piston back, against the force of the primary piston spring, and the rear brake pressure. If enough fluid returns, the centre valve opens, and allows fluid to return to the reservoir.

If fluid is returned from the rear brake circuit, the secondary and primary pistons tend to be forced apart.

The amount of fluid that returns to the master cylinder is determined by the degree of anti-lock braking control. With approximately 4 to 6 ABS control cycles per second, the rapid changes in pressure cause pulsations that can be felt by the driver at the brake pedal.

Hydraulic control unit

Summary

The hydraulic control unit, or modulator, executes commands in the form of electrical signals from the ABS Control Module. It uses solenoid valves to change the hydraulic pressure in the brake circuit.

The ABS Control Module, or ECU, sends commands in the form of electrical signals to the hydraulic control unit. This unit executes the commands, using three solenoid valves connected in series with the master cylinder and the brake circuits - one valve for each front wheel hydraulic circuit, and one for both of the rear wheels.

In normal, non-ABS braking, brake pedal force is transmitted to the master cylinder, then through the solenoid valve to the brake unit at the wheel. When the signals from the wheel speed sensor show no tendency for the wheel to lock up, the ECU does not send any control current to the solenoid coil. The solenoid valve is not energized, and the hydraulic pressure from the master cylinder is supplied to the brake unit at the wheel.

When the control unit detects any lock-up tendency, perhaps from too-rapid wheel deceleration, it sends a command current to the solenoid coil. This causes the armature and valve to move upward, and isolate the brake circuit from the master cylinder. That keeps the pressure between the solenoid and the brake circuit constant - whether or not the master cylinder hydraulic pressure rises.

If the sensors signal continuing excessive wheel deceleration, the Control Module sends a larger current to the solenoid valve. This lowers the braking pressure by moving the armature up further, opening a passage from the brake circuit to an accumulator - a temporary reservoir for any brake fluid that flows out of the wheel brake cylinders because of the fall in pressure. A return pump sends this brake fluid back to the master cylinder.

If the sensors then signal that the lower pressure has allowed the wheel to speed up, the ECU stops all command current, which de-energizes the solenoid valve. The pressure rises, and the wheel is again slowed down.

Whatever the phase of operation, pressure in the circuit can never rise above master cylinder pressure.

Wheel speed sensors

Summary

Wheel speed sensors consist of a toothed rotor and a pickup. Wheel rotation sends input signals to the ECU, which processes them and controls the hydraulic control unit.

A wheel sensor consists of a toothed rotor that rotates with the wheels, and a pickup. As each tooth of the rotor passes the pickup, a small voltage is induced in the pickup.

These pulses are sent as input signals to the electronic control unit which processes them, to operate the hydraulic control unit.

ABS electronic control unit

Summary

The ECU receives signals from various sources. The brake pedal, the ignition system, and wheel speed sensors control the hydraulic control unit and anticipate wheel lock.

The electronic control unit receives signals from different sources. A switch at the brake pedal provides a brake-operating signal. Another in the ignition system signals the engine is operating. This sets off the automatic check the ABS conducts every time the engine starts.

Another input is from the wheel speed sensors. These signals are used to control the hydraulic control unit and anticipate wheel lock. If a wheel starts to lock, the electronic control unit operates the solenoid valves to reduce hydraulic pressure appropriately.

ABS brake system

Summary

The primary components of the antilock braking system are: The electronic control unit, hydraulic control unit or modulator, power booster & master cylinder assembly and wheel sensor unit.

ABS brakes

The antilock braking system is designed to prevent wheels locking or skidding, no matter how hard brakes are applied, or how slippery the road surface.

The primary components of the ABS braking system are:

Electronic control unit (ECU)

Hydraulic control unit or modulator

Power booster & master cylinder assembly

Wheel sensor unit

Electronic control unit (ECU)

The ECU is located inside the vehicle. It receives signals from the sensors in the circuit and controls the brake pressure at the road wheels according to the data analyzed by the Unit.

Hydraulic control unit or modulator

The location of the Hydraulic Control Unit, or Modulator, varies from manufacturer to manufacturer. Some locate it under the fender or hood.

It receives operating signals from the ECU to apply or release the brakes under ABS conditions.

Power booster & master cylinder assembly

The power booster and master cylinder assembly is mounted on the firewall and is activated when the driver pushes down on the brake pedal. It provides the power assistance required during braking.

Wheel sensor unit

The wheel sensor unit consists of a tooth rotor that rotates with the road wheels and a pick-up that is located in the wheel hub.

Checking & adjusting brake fluid

Summary

The objective of this procedure is to show you how to check and adjust brake fluid level. Brake fluid will absorb moisture rapidly. Always replace the cover or lid as soon as possible.

Part 1. Preparation and safety

Objective

Check and adjust brake fluid level.

Personal safety

Whenever you perform a task in the workshop you must use personal protective clothing and equipment that is appropriate for the task and which conforms to your local safety regulations and policies. Among other items, this may include:

Work clothing – such as coveralls and steel-capped footwear

Eye protection – such as safety glasses and face masks

Ear protection – such as earmuffs and earplugs

Hand protection - such as rubber gloves and barrier cream

Respiratory equipment - such as face masks and valved respirators

If you are not certain what is appropriate or required, ask your supervisor.

Safety check

Never use any petroleum or mineral based products, such as gasoline, kerosene etc, to clean a braking system or its components. They are not compatible and will result in a failure of the braking system and its components. This may result in injury to the passengers or damage to the vehicle.

Make sure that you understand and observe all legislative and personal safety procedures when carrying out the following tasks. If you are unsure of what these are, ask your supervisor.

Points to note

Brake fluid can damage the paintwork by softening the base paint.

Brake fluid has a hygroscopic nature; it will absorb moisture rapidly. Always replace the cover or lid as soon as possible.

The higher the D.O.T. number of the brake fluid, the higher its boiling point.

Do not mix any D.O.T. 5 silicone-based brake fluid with a D.O.T. 3 or 4 glycol-based fluid because they are incompatible.

As glycol-based brake fluid absorbs moisture its boiling point is lowered. This can cause the phenomenon known as 'brake fade'.

Brake fluid is stored in the master cylinder. If you are unsure of its location, consult the vehicle's shop manual or the owner's manual.

If your vehicle has antilock brakes, consult the owner's manual before filling the cylinder.

If brake fluid splashes into your eyes, rinse it out with tap water immediately.

Do not swallow brake fluid. It is toxic.

Brake fluid reservoirs will indicate the maximum and minimum levels with a marker on the side or level bars inside the container.

Part 2: Step-by-step instruction

1. Check brake fluidWipe around the master cylinder top cover to prevent any dirt from entering the system. Open the top of the master cylinder, by removing the plastic lid, or by prying the retaining clamp from a metal cover. Look inside the master cylinder. The fluid should be up to the “FULL” line on the side of the cylinder or within half an inch of the top of each chamber.

2. Adjust brake fluidAdd the manufacturer’s recommended brake fluid only if needed. Replace the cover and check that it is fully seated.

3. Final inspectionCheck for any leaks around the master cylinder with a flashlight. Dilute any brake fluid that may have been spilt with fresh clean water. Do not rub the fluid with a cloth.

Replacing brake fluid

Summary

Most manufacturers recommend that brake fluid be changed at least every two years. The objective of this procedure is to show you how to test and replace brake fluid in accordance with manufacturer's recommendations.

Part 1. Preparation and safety

Objective

Test and replace brake fluid in accordance with manufacturer's recommendations.

Personal safety

Whenever you perform a task in the workshop you must use personal protective clothing and equipment that is appropriate for the task and which conforms to your local safety regulations and policies. Among other items, this may include:

Work clothing - such as coveralls and steel-capped footwear

Eye protection - such as safety glasses and face masks

Ear protection - such as earmuffs and earplugs

Hand protection - such as rubber gloves and barrier cream

Respiratory equipment - such as face masks and valved respirators

If you are not certain what is appropriate or required, ask your supervisor.

Safety check

Never use any petroleum or mineral based products, such as gasoline, kerosene etc, to clean a braking system or its components. They are not compatible and will result in a failure of the braking system and its components. This may result in injury to the passengers or damage to the vehicle.

If brake fluid splashes into your eyes, rinse it out with tap water immediately.

Do not swallow brake fluid. It is toxic.

Make sure that when you are bleeding the brakes, you do not empty the brake fluid reservoir or you will cause internal damage to the master cylinder.

Make sure that you understand and observe all legislative and personal safety procedures when carrying out the following tasks. If you are unsure of what these are, ask your supervisor.

Points to note

Brake fluid can damage the paintwork by softening the base paint.

Brake fluid has a hygroscopic nature; it will absorb moisture rapidly.

As moisture is absorbed, the boiling point is lowered. This can cause the phenomenon known as 'brake fade'.

Always replace the reservoir cover or lid as soon as possible.

Do not use brake fluid that has been left in an open container.

Brake fluid types are identified by their D.O.T. number. The higher the D.O.T. number, the higher its boiling point.

Most vehicles use either D.O.T. 3 or D.O.T. 4 fluid. These fluids are both glycol-based but differ in specifications. The reservoir cover or cap will usually state the type of fluid to be used.

A D.O.T. 5 is synthetic fluid and normally referred to as 'silicone' brake fluid; it's usually colored purple for easy identification and it's not recommended for vehicles fitted with ABS.

Do not mix D.O.T. 5 brake fluid with a D.O.T. 3 or 4 glycol-based fluid. They are incompatible.

The majority of today's braking systems use a combination of dissimilar metals. Manufacturers use aluminum in pistons and housings, steel in some wheel cylinders and brake lines.

When moisture mixes with brake fluid a galvanic action can occur. Galvanic action is similar to the operation of a battery. The more moisture in brake fluid, the higher the galvanic reading and the greater the corrosion it causes.

If you need to bleed your brakes, you may need to replace your brake fluid.

When bleeding the brakes, it will be necessary to top up the brake fluid after bleeding each brake station.

Most manufacturers recommend that brake fluid be changed at least every two years. Consult your owner's manual for the specified intervals for the model you are working on and the type of replacement brake fluid recommended.

There are a number of different brake bleeding methods. Some repair shops will use a 'pressure bleeder' while others may have a 'vacuum brake bleeder'. Manual bleeding is discussed in this procedure.

Manufacturers recommend that when bleeding brakes; push the brake pedal to the floor. If you are changing brake fluid on an older vehicle, or if you are uncertain of the vehicle's service history, it is wise to depress the pedal through its normal range of movement. This will prevent the components in the master cylinder from coming into contact with contaminants.

Brake assemblies, either disc or drum, are often referred to as braking stations.

Part 2: Step-by-step instruction

1. Check for galvanic reactionRemove the brake fluid reservoir cap, and set the DVOM to read voltage on the lowest voltage range of the meter. Place one of the DVOM probes into the brake fluid, and the other against the body of the master cylinder. The voltage should be no greater than 0.3 volts. If the voltage is higher, this indicates a galvanic reaction, and means that there is an unacceptable level of moisture in the brake fluid.

2. Remove the excess brake fluidUse an old hydrometer or similar device to suck out the contaminated fluid from the master cylinder reservoir. Place the discarded brake fluid into a container that can be disposed of in an environmentally friendly manner.

3. Top up with new fluidTop up the master cylinder reservoir with clean fluid until it reaches the “FULL” line. Close the brake fluid container as quickly as possible to prevent contamination and evaporation of the fluid.

4. Manually bleed the brake systemOpen the bleeder screw that is the furthest from the master cylinder and ask an assistant to slowly push the brake pedal down. With a clear bleeder hose inserted into a jar, you can observe the air bubbles coming out. Close the bleeder screw off, and have the assistant slowly release the pedal. Continue this process until there are no more air bubbles coming out of the brake station and the new brake fluid is present. Close off the bleeder screw and tighten it to the manufacturers specifications. Repeat this procedure for each of the other brake stations, moving closer to the master cylinder.

5. Inspect and testHave your supervisor inspect each of the brake stations to ensure that there are no leaks and the bleeder screws are closed correctly. A simple way of doing this is to ask the assistant in the vehicle to place their foot on the brake pedal, as if they were applying the brakes when driving the vehicle normally. Now you can check all the bleed points and hose connections for leaks while the assistant is maintaining the pressure on the system, and thus any fluid that could be leaking as a result of a loose joint in the components. After your supervisor is satisfied that it is all correct, lower the vehicle. Carry out a final check for any leaks around the master cylinder or brake stations. Replace the master cylinder cover and check to see that it is fully seated. Dilute any brake fluid that may have been spilt with fresh clean water. Ask your supervisor to check the brake system pedal pressure, fluid levels, and visual signs of leaks. When the supervisor is satisfied, the vehicle should be road tested by a qualified person to ensure the integrity of the braking system.

Checking brake pads

Summary

You should check the disc brakes and disc brake linings every 15,000 kilometers (10,000 miles). The objective of this procedure is to show you how to check brake pads and calipers as well as check and measure rotors.

Part 1. Preparation and safety

Objective

Check brake pads and calipers. Check and measure rotors.

Personal safety

Whenever you perform a task in the workshop you must use personal protective clothing and equipment that is appropriate for the task and which conforms to your local safety regulations and policies. Among other items, this may include:

Work clothing - such as coveralls and steel-capped footwear

Eye protection - such as safety glasses and face masks

Ear protection - such as earmuffs and earplugs

Hand protection - such as rubber gloves and barrier cream

Respiratory equipment - such as face masks and valved respirators

If you are not certain what is appropriate or required, ask your supervisor.

Safety check

Make sure that you understand and observe all legislative and personal safety procedures when carrying out the following tasks. If you are unsure of what these are, ask your supervisor.

Points to note

Most vehicles have four-wheel disc brakes.

You should check the disc brakes and disc brake linings every 15,000 kilometers (10,000 miles).

After checking the brake pads, replace the road wheel but don't put lug nuts into the socket of the impact wrench and power the lug nuts on; this practice can lead to the wheel nuts going on cross-threaded.

When inspecting the caliper be very careful because the caliper may be hot if the car has been driven recently.

Part 2: Step-by-step instruction

1. Check brake caliperMake sure the caliper is cool to the touch, then grasp it and try to move it to make sure it’s mounted securely.

2. Check brake padsLook through the dust shield on the caliper and look at the brake pads. Check the linings on the brake pads. If they are too worn, they may have to be replaced. The easiest way to estimate the wear is to look at the thickness of the backing plate of the pad, and, if the thickness of the remaining brake lining surface of the pad is the same or less than that of the thickness of the backing plate, the pads should be replaced. If the linings have worn so much that they are running on the pads backing plates, have your supervisor inspect the system, as the disc may have to be reground.

3. Check and measure the rotorCheck the visible part of the rotor for rust, scoring, and uneven wear. Check the shop manual for the specifications of this rotor and measure its thickness with an outside micrometer. If the reading is outside the manufacturers specification, report it to your supervisor. Repeat these procedures for the other three wheels.

Replacing brake pads

Summary

The objective of this procedure is to show you how to replace brake pads and refit according to the manufacturer's recommended procedure. Remember to remove some of the brake fluid from the master cylinder before you raise the vehicle.

Part 1. Preparation and safety

Objective

Replace brake pads and refit according to the manufacturer's recommended procedure.

Personal safety

Whenever you perform a task in the workshop you must use personal protective clothing and equipment that is appropriate for the task and which conforms to your local safety regulations and policies. Among other items, this may include:

Work clothing - such as coveralls and steel-capped footwear

Eye protection - such as safety glasses and face masks

Ear protection - such as earmuffs and earplugs

Hand protection - such as rubber gloves and barrier cream

Respiratory equipment - such as face masks and valved respirators

If you are not certain what is appropriate or required, ask your supervisor.

Safety check

Make sure that you understand and observe all legislative and personal safety procedures when carrying out the following tasks. If you are unsure of what these are, ask your supervisor.

Points to note

Before you raise the vehicle, you will need to remove some of the brake fluid from the master cylinder. Using a siphon, remove enough fluid to cause the reservoir level to drop well below the fill mark. This allows the fluid in the lines to return to the reservoir without overflowing when you retract the caliper pistons for the new pads.

During inspection, the flexible brake hose will need to be supported because it is still attached to the caliper. The easiest way to do this is by fastening a length of wire around the caliper and attaching it to a suspension component.

Do not let the caliper hang unsupported because you may damage the flexible brake hose.

Part 2: Step-by-step instruction

1. Remove brake fluidBefore you raise the vehicle, you will need to remove some of the brake fluid from the master cylinder to allow for fluid return into the master cylinder reservoir when you install the new brake pads. Using a siphon, remove enough fluid so that the reservoir level is well below the fill mark.

2. Raise the vehicle and remove wheel assemblyRaise the vehicle to a comfortable working height. Remove the wheel assembly.

3. Inspect the brake assemblyCheck for any signs of fluid leaks or grease leakage from the hub seal. Check for any signs of a cracked or damaged rotor. Inspect the brake lines to see if there are any signs of leakage or deterioration of the flexible hoses.

4. Remove the brake padsLoosen and remove the caliper location pins or bolts. Lift and rotate the caliper away from the rotor. Ensure that the caliper is not hanging unsupported. Secure and support the caliper by fastening a length of wire around it and attaching it to a suspension component. Remove the worn brake pads from the caliper by pulling them from their guide locations. It is good practice to lightly skim the rotors when new pads are to be installed, so arrange with your supervisor to have the rotor machined. Carry out a run out test with a dial gauge in addition to measuring rotor thickness with an outside micrometer. This will determine the amount of material to be skimmed. Mount the dial gauge fixture to a fixed point in relation to the rotor, so that when the rotor is turned it shows how much run out variation there is in the disc. Check the amount of run-out against the manufacturers specifications and report any excessive tolerances in your findings to your supervisor .

5. Check the slide or locating pinsCheck the locator pins, sleeves, and insulators for any signs of wear or binding. With the sliding caliper type, ensure the caliper can slide freely on the runners.

6. Retract the pistonTo fit new pads, you will need to retract the caliper piston back into its housing. One of the most common methods is to fit a block of wood and a ‘C’ or ‘G’ clamp over the piston. Tighten the clamp to retract the piston back into the housing. As the piston moves back, it will displace brake fluid back into the master cylinder reservoir. Remove the block of wood and clamp after the piston has been retracted.

7. Install the new brake padsCompare the new brake pads to the originals to confirm that they are the correct pads to install. The backing plates should be exactly the same shape. Install the new pads into the caliper. Slide the caliper assembly onto the rotor, and align the mounting or locating pins. Refer to the shop manual for the correct procedure and specifications for installing the pins or bolts. Install the pins or bolts and secure them in place. Torque the bolts or retainers to the correct specification.

8. Check/remove brake system air and refit wheelsUsing the correct procedure bleed the brakes and refit the wheel assemblies.

Removing & replacing a rotor

Summary

Before working on a vehicle's brakes, attach a safety notice to the vehicle to prevent people attempting to operate the brakes. The objective of this procedure is to show you how to safely remove and reinstall a disc brake rotor.

Part 1. Preparation and safety

Objective

Safely remove and reinstall a disc brake rotor.

Personal safety

Whenever you perform a task in the workshop you must use personal protective clothing and equipment that is appropriate for the task and which conforms to your local safety regulations and policies. Among other items, this may include:

Work clothing - such as coveralls and steel-capped footwear

Eye protection - such as safety glasses and face masks

Ear protection - such as earmuffs and earplugs

Hand protection - such as rubber gloves and barrier cream

Respiratory equipment - such as face masks and valved respirators

If you are not certain what is appropriate or required, ask your supervisor.

Safety check

Brake components can be very hot even though they do not appear to be. Take precautions against burns when working on them.

Always follow the manufacturer’s procedure and specifications.

Do not allow the disc calliper to hang by the brake hose. Support it with a piece of wire.

Properly clean and dispose of brake dust.

Seat the brake pads in position before driving the vehicle.

Make sure that you understand and observe all legislative and personal safety procedures when carrying out the following tasks. If you are unsure of what these are, ask your supervisor.

Points to note

Clean the mating surfaces of the hub and rotor before assembly.

If the rotor is being replaced with a new rotor, clean it according to the installation instructions.

Test the brake pedal to seat the brake pads before driving the vehicle.

Part 2: Step-by-step instruction

1. Attach safety noticeBefore working on a vehicle’s brakes, attach an appropriate label to the vehicle to prevent accidental actuation of the brake caliper while it is removed from the hub.

2. Prepare the vehicleSafely raise the vehicle and remove the wheel.Remove the brake pads and caliper; making sure the caliper is located safely away from the rotor.

3. Remove the rotorRemove the rotor by unbolting the securing device and sliding it away from the wheel hub.

4. Prepare the new rotorBefore installing the new rotor, clean it thoroughly in a component washer to remove any storage preservative agents.

5. Fit the rotorClean any dirt, rust and debris from the hub mating surface. Secure the rotor to the hub and rotate it by hand to ensure it rotates freely.

6. Reassemble the wheelRefit the brake caliper and disc pads. Apply the brakes several times to readjust the caliper pistons. Refit the road wheel & safely lower the vehicle.

7. Road test the vehicleAsk your supervisor to check the brake system pedal pressure, fluid levels, and visual signs of leaks. When the supervisor is satisfied, the vehicle should be road tested by a qualified person to ensure the integrity of the braking system.

Removing & replacing a rotor

Summary

Before working on a vehicle's brakes, attach a safety notice to the vehicle to prevent people attempting to operate the brakes. The objective of this procedure is to show you how to safely remove and reinstall a disc brake rotor.

Part 1. Preparation and safety

Objective

Safely remove and reinstall a disc brake rotor.

Personal safety

Whenever you perform a task in the workshop you must use personal protective clothing and equipment that is appropriate for the task and which conforms to your local safety regulations and policies. Among other items, this may include:

Work clothing - such as coveralls and steel-capped footwear

Eye protection - such as safety glasses and face masks

Ear protection - such as earmuffs and earplugs

Hand protection - such as rubber gloves and barrier cream

Respiratory equipment - such as face masks and valved respirators

If you are not certain what is appropriate or required, ask your supervisor.

Safety check

Brake components can be very hot even though they do not appear to be. Take precautions against burns when working on them.

Always follow the manufacturer’s procedure and specifications.

Do not allow the disc calliper to hang by the brake hose. Support it with a piece of wire.

Properly clean and dispose of brake dust.

Seat the brake pads in position before driving the vehicle.

Make sure that you understand and observe all legislative and personal safety procedures when carrying out the following tasks. If you are unsure of what these are, ask your supervisor.

Points to note

Clean the mating surfaces of the hub and rotor before assembly.

If the rotor is being replaced with a new rotor, clean it according to the installation instructions.

Test the brake pedal to seat the brake pads before driving the vehicle.

Part 2: Step-by-step instruction

1. Attach safety noticeBefore working on a vehicle’s brakes, attach an appropriate label to the vehicle to prevent accidental actuation of the brake caliper while it is removed from the hub.

2. Prepare the vehicleSafely raise the vehicle and remove the wheel.Remove the brake pads and caliper; making sure the caliper is located safely away from the rotor.

3. Remove the rotorRemove the rotor by unbolting the securing device and sliding it away from the wheel hub.

4. Prepare the new rotorBefore installing the new rotor, clean it thoroughly in a component washer to remove any storage preservative agents.

5. Fit the rotorClean any dirt, rust and debris from the hub mating surface. Secure the rotor to the hub and rotate it by hand to ensure it rotates freely.

6. Reassemble the wheelRefit the brake caliper and disc pads. Apply the brakes several times to readjust the caliper pistons. Refit the road wheel & safely lower the vehicle.

7. Road test the vehicleAsk your supervisor to check the brake system pedal pressure, fluid levels, and visual signs of leaks. When the supervisor is satisfied, the vehicle should be road tested by a qualified person to ensure the integrity of the braking system.

Checking wheel cylinders

Summary

Some vehicles still have drum brakes or a combination of discs and drums. Passenger cars with this combination will have the drum brakes on the rear. Brake linings should be checked approximately every 15,000 kilometers (10,000 miles). The objective of this procedure is to show you how to check wheel cylinders, drum brake linings and measure brake drums.

Part 1. Preparation and safety

Objective

Check wheel cylinders, drum brake linings and measure brake drums.

Personal safety

Whenever you perform a task in the workshop you must use personal protective clothing and equipment that is appropriate for the task and which conforms to your local safety regulations and policies. Among other items, this may include:

Work clothing - such as coveralls and steel-capped footwear

Eye protection - such as safety glasses and face masks

Ear protection - such as earmuffs and earplugs

Hand protection - such as rubber gloves and barrier cream

Respiratory equipment - such as face masks and valved respirators

If you are not certain what is appropriate or required, ask your supervisor.

Safety check

Make sure that you understand and observe all legislative and personal safety procedures when carrying out the following tasks. If you are unsure of what these are, ask your supervisor.

Points to note

Brake linings should be checked approximately every 15,000 kilometers (10,000 miles).

Check under the dust cap to make sure that the wheel cylinder is not leaking. If you find a leaking wheel cylinder, report it to your supervisor.

Part 2: Step-by-step instruction

1. Remove the drumsSome vehicles still have drum brakes or a combination of discs and drums. Passenger cars with this combination will have the drum brakes on the rear. Remove the brake drums. Be careful when you touch the brakes, they may be hot if the vehicle has been driven recently.

2. Check wheel cylinders and liningsInspect the external portion of the wheel cylinder. If the backing plate is covered with an oily substance you will need to determine if it is brake fluid or grease. Brake fluid will wash away with water so try washing it off. If it does not wash off then it is probably grease from a faulty seal. Carefully lever the rubber dust seals back, and inspect the inner portion of the cylinder for any signs of leakage.

3. Check the brake shoesCheck the thickness of the brake shoe lining. You will need to consult the shop manual for the allowable minimum thickness. If the thickness is at this point or close to the minimum you must inform your supervisor. Also, if the brake shoe linings are covered in oil or grease they must be replaced. Report any signs of contamination or leakage to your supervisor.

4. Check and measure the drumVisually inspect the drum-braking surface for scoring. Any large score marks mean that the drum will have to be machined or replaced. Measure the inner diameter of the brake drum at several points around the circumference. Consult the shop manual for the specifications for an out-of-round condition. If the reading is outside the manufacturer’s specification, report it to your supervisor.

5. Reassemble the drum brakeReassemble the drum brake. Repeat the procedure for the other drum brake assemblies .

Replacing brake linings

Summary

Linings are designed to withstand extreme heat generation during braking. The linings must be able to withstand these temperatures and still maintain their braking efficiency. The objective of this procedure is to show you how to remove and replace drum brake linings.

Part 1. Preparation and safety

Objective

Remove and replace drum brake linings.

Personal safety

Whenever you perform a task in the workshop you must use personal protective clothing and equipment that is appropriate for the task and which conforms to your local safety regulations and policies. Among other items, this may include:

Work clothing - such as coveralls and steel-capped footwear

Eye protection - such as safety glasses and face masks

Ear protection - such as earmuffs and earplugs

Hand protection - such as rubber gloves and barrier cream

Respiratory equipment - such as face masks and valved respirators

If you are not certain what is appropriate or required, ask your supervisor.

Safety check

Never use any petroleum or mineral based products, such as gasoline, kerosene etc, to clean a braking system or its components. Doing so can result in a failure of the braking system and its components. This may result in serious injury, or damage to the vehicle.

Make sure that you understand and observe all legislative and personal safety procedures when carrying out the following tasks. If you are unsure of what these are, ask your supervisor.

Points to note

Linings are designed to withstand extreme heat generation during braking. The linings must be able to withstand these temperatures and still maintain their braking efficiency.

Oil or grease contaminants on the lining can have an adverse effect on the co-efficient of friction of the linings.

If you need to bleed your brakes, you may need to replace your brake fluid.

If you are bleeding brakes on an older vehicle, or you don't know the vehicle's service history, it is good practice not to fully depress the brake pedal during bleeding.

Part 2: Step-by-step instruction

1. Prepare the vehicleBefore you raise the vehicle you will need to remove some of the brake fluid from the master cylinder. Using a siphon, remove enough fluid to cause the reservoir level to be below the fill mark. This allows for a fluid expansion when you retract the wheel cylinder pistons for the new linings. Raise the vehicle using the correct procedure to a comfortable working height and remove the wheel assembly.

2. Remove and inspect the brake drumRemove the brake drum taking care to remove any locating screws and back off any manual adjusters if fitted during the removal process. Check for any signs of fluid leaks or grease leakage from a hub or axle shaft seal. Check the brake drum for any signs of cracks or other damage.

3. Inspect the brake linesInspect the brake lines to see if there are any signs of leakage or deterioration of flexible and/or steel hoses. If replacement is required report this to your supervisor.

4. Remove the brake shoes and liningsInstall a brake cylinder clamp onto the wheel cylinder. This will prevent the piston coming out when you release the brake shoes. Check with the shop manual for the correct service tools you need to release the springs and clips. Using the correct tool, release the retaining clips and disconnect the return springs. Remove the worn brake shoes and their linings from the backing plate.

5. Check the retaining clips and return springsCheck the condition of the return springs making sure they have not got spread coils that will reduce their efficiency. Check that the retaining clips are not damaged and will keep the brake shoes in position.

6. Install the new brake shoesCompare the new brake shoes and their linings to the original ones to make sure they are the correct replacements. Referring to the shop manual, install the brake shoes onto the backing plate. Install the return springs and retaining clips in accordance with the manufacturers specification and direction.

7. Refit the brake drumAs the return spring tension takes effect, the wheel cylinder piston should move back into

the cylinder. If it doesn’t you may have to manually retract the piston. Now refit the brake drum. Prepare the cylinder for bleeding using a clear bleeder hose and a jar with a small amount of brake fluid in the base. Open the bleeder screw and have your assistant slowly push the brake pedal down and observe the air bubbles coming out. Close the bleeder screw off, and have the assistant slowly release the pedal. Manually adjust the brakes if necessary to ensure that the drum rotates without binding. Refit the wheel and tire assembly using the correct procedure. Repeat this process for each of the other drum brake stations.

8. Bleed system if required and testIf at the completion of the replacement process the brake pedal feels “spongy”, you should bleed the system of air from all of the brake stations. Using the correct procedure to do this, and at the completion of the job, you should recheck the system to ensure that a firm pedal is achieved, and that the wheels cannot be rotated when the brake pedal is applied. Have your supervisor check the system and verify the repair.

Adjusting a park brake cable

Summary

There are two main ways of adjusting park brakes on vehicles; one method is done from underneath the vehicle, and the other from inside. A vehicle will only be fitted with one method of adjustment.The objective of this procedure is to show you how to check and adjust park brake cable following the manufacturer's procedure.

Part 1. Preparation and safety

Objective

Check and adjust park brake cable following the manufacturer's procedure.

Personal safety

Whenever you perform a task in the workshop you must use personal protective clothing and equipment that is appropriate for the task and which conforms to your local safety regulations and policies. Among other items, this may include:

Work clothing - such as coveralls and steel-capped footwear

Eye protection - such as safety glasses and face masks

Ear protection - such as earmuffs and earplugs

Hand protection - such as rubber gloves and barrier cream

Respiratory equipment - such as face masks and valved respirators

If you are not certain what is appropriate or required, ask your supervisor.

Safety check

If the vehicle is being raised on a floor jack, safety stands must be used to prevent the possibility of the vehicle coming down while you are under it. Normally, the vehicle will be serviced on a hoist.

Make sure that you understand and observe all legislative and personal safety procedures when carrying out the following tasks. If you are unsure of what these are, ask your supervisor.

Points to note

The park brake must operate independently of the service brakes.

Some manufacturers use an equalizer bracket to make sure that each brake station receives the same or equal pressure.

The park brake can operate off the front or rear brakes. In some applications it operates off the back of the transmission output shaft.

Part 2: Step-by-step instruction

1. Prepare the vehicleThere are two main ways of adjusting park brakes on vehicles; one method is done from underneath the vehicle, and the other from inside. A vehicle will only be fitted with one method of adjustment. The park brake is also known as the emergency brake. It must be able to operate independently from the service brakes. To check the brakes serviceability raise the car on a jack or hoist, making sure that the car is secure when raised. If you use a floor jack, make sure you support the vehicle with axle stands under the vehicle. Make sure the parking brake is off. Ensure the service brakes have been adjusted and are operative before carrying out any park brake adjustments. Check the shop manual for the correct adjustment travel for the vehicle.

2. Check/adjust parking brake cable (under vehicle)Trace the thin steel cables that run from each of the rear wheels until they meet under the car. Where they intersect there is normally a threaded rod with an adjusting nut located in a trunion. Loosen the jam nut, and turn the adjusting nut screw until the cables tighten, and then tighten the jam nut to hold the rod in place. Rotate the rear wheels to ensure they are not binding.

3. Check/adjust parking brake cable (in vehicle)Remove the protective cover from around the park brake lever or handle. Loosen the jam nut on the adjusting rod and turn the adjusting screw until the cables tighten, and then tighten the jam nut to hold the screw in place. Replace the cover over the cables to prevent any dirt entering into the vehicle. Rotate the rear wheels to ensure they are not binding.

4. Test the vehicleLower the car, and then test it by parking on an incline, by applying the hand brake and counting the clicks to ensure that lever movement is in accordance with specifications and to make sure the brake holds the vehicle properly. While you’re driving note whether the parking light emergency light is on, if so look at the manufacturers recommendations for the correct method to adjust the parking brake sensor unit.