Chpt 70 Hydraulic Valves and Switches

Hydraulic Valves and Switches

. After studying Chapter 70, the reader will be able to: Prepare for ASE Brakes (AS) certification test content area "A" (Hydraulic System Diagnosis and Repair). • Describe the operation of a residual check valve. • Explain how a proportioning valve works. • Discuss the need and use of a metering valve. • List testing procedures used to test hydraulic valves. • Describe how the brake fluid level and brake light switches work.

brake fluid level sensor • brake light switch • combination valve • electronic brake proportioning • expander • height-sensing proportioning valve • metering valve • pressure-differential switch • proportioning valve • residual check valve • slope • split point

MASTER CYLINDER FLUID OUTLET

RESIDUAL CHECK VALVE A residual check valve has been used on some drum brake sys· tems to keep a slight amount of pressure on the entire hydraulic system for drum brakes (5 to 12 PSI). See Figure 70-1.

This residual check valve is located in the master cylinder at the ouDet for the drum brakes. The check ball and spring in the residual check valve permit all the brake fluid to return to the master cylinder until the designated pressure is reached .

This slight pressure prevents air leaks from entering into the hydraulic system in the event of a small hole or leak. With a low

. pressure kept on the hydraulic system, any small hole will cause fluid to leak out rather than permit air to enter the system. This slight pressure also keeps the wheel cylinder sealing cups tight against the inside wall of the wheel cylinder. See Figure 70-2.

Residual check valves are often not used on late model vehicles equipped with front disc/ rear drum brakes. The residual check valve has been eliminated by equipping the wheel cylinder internal spring with a sealing cup expander to prevent sealing cup lip collapse. See Figure 70-3.

PRESSURE·DIFFERENTIAL SWITCH (BRAKE WARNING SWITCH) A pressure-differential switch is used on all vehicles built after 1967 with dual master cylinders to warn the driver of a loss of pressure in one of the two separate systems by lighting the dashboard red brake warning indicator lamp. See Figure 70-4.

SPRINGY

RESIDUAL CHECK VALVE

FLUID RETURNING TO MASTER CYLINDER

Figure 70-2 The momentary drop in pressure created when the brakes are released can draw air into the hydraulic system.

CUP SEAL

Figure 7il-3 The use of cup expanders is the main reason why residual check valves are not used in most braking systems today.

~TUBING SEAT

Figure 70-1 Most residual check valves are located under the tubing seals in the master cylinder outlet ports. Figure 70-4 Ared brake warning lamp.

840

The brake lines from both the front and the rear sections of the master cylinder are sent to this switch, which lights the brake warning indicator lamp in the event of a "difference in pressure" between the two sections. See Figure 70-5.

A failure in one part of the brake system does not result in a failure of the entire hydraulic system. After the hydraulic system has been repaired and bled, moderate pressure on the brake pedal will center the piston in the switch and turn off the warn· ing lamp.

Chapter 70: Hydraulic Valves and Switches 841

If the lamp remains on, it may be necessary to do the following:

1. Apply light pressure to the brake pedal. 2. Momentarily open the bleeder valve on the side that did not have

the failure.

This procedure should center the pressure-differential switch valve in those vehicles that are not equipped with self-centering springs. See Figure 70-6.

~':::====I F===~

;l A LEAK IN EITHER SYSTEM DROPS i----' PRESSURE TO THAT SYSTEM I

FRONT BRAKE PRESSURE ---,..--u.......J.J REAR BRAKE PRESSURE IS APPLIED HERE IS APPLIED HERE

SPRING-LOADED WARNING SWITCH

THE TRIGGER IS PUSHED UPWARD TO CLOSE SWITCH AND ILLUMINATE --------i~11I

BRAKE WARNING LAMP ON INSTRUMENT PANEL

THE PISTON MOVES TOWARD THE REDUCED PRESSURE SIDE

Figure 7G-5 Aleak in the hydraulic system causes unequal pressures between the two different brake circuits.This difference in pressures causes the plunger inside the pressure-differential switch to move, which completes the electrical circuit for the red brake warning lamp.

LOWERED RAISED SWITCH SWITCH GROUNDED PLUNGER PLUNGER TERMINAL

STUD

PISTON CENTER

PISTON CENTER

PISTON CENTER

Figure 7~ The pressure-differential switch piston is used to provide the electrical ground for the red brake warning light circuit.

842 SECTION XI

BRAKE FLUID LEVEL SENSOR SWITCH Many master cylinders, especially systems that are a diagonal split, usually use a brake fluid level sensor or switch in the master cylinder reservoir. This sensor will light the red "brake" warning lamp on the dash if low brake fluid level is detected. A float·type sensor or a magnetic reed switch are commonly used and provide a complete electrical circuit when the brake fluid level is low. After refilling the master cylinder reservoir to the correct level, the red "brake" warning lamp should go out. See Figures 70-7 and 70-8.

DIAGNOSING A RED "BRAKE" DASH WARNING LAMP Activation of the red brake dash warning lamp can be for anyone of several reasons:

1. Parking Brake "On." The same dash warning lamp is used to warn the driver that the parking brake is on.

2. Low Brake Fluid. This lights the red dash warning lamp on vehi· cles equipped with a master cylinder reservoir brake fluid level switch.

3. Unequal Brake Pressure. The pressure·differential switch is used on most vehicles with a front/ rear brake split system to warn the driver whenever there is low brake pressure to either the front or rear brakes.

FLUID LEVEL SENSOR

FIXED CONTACTS

FLOAT MASTER CYLINDER

Figure 7~7 A movable contact brake fluid level switch.

NOTE: Brake systems use either a pressure·differential switch or a low brake fluid switch to light the dash red "brake" lamp, but not both.

The most likely cause of the red "brake" warning lamp being on is low brake fluid caused by a leaking brake line, wheel cylinder, or caliper. Therefore, the first step in diagnosis is to determine the cause of the lamp being on, then to repair the problem.

Step #1 Check the level of the brake fluid. If low, carefully inspect the entire hydraulic brake system for leaks and repair as necessary.

Step #2 Disconnect the wire from the pressure-differential switch. If the lamp is still "on," the problem is due to the parking brake lever switch being "on" or grounded, or the wire going to the switch is shorted to ground. If the red brake warning lamp is "off" after being disconnected from the pressure·differential switch, then the problem is due to a hydraulic failure [a low pressure in either the front or the rear system that creates a difference in pressure of at least 150 PSI).

NOTE: Many Japanese vehicles energize the relay that turns off the red "brake" warning lamp from the output terminal of the alternator. If a quick inspection of the brake system seems to indicate that everything is okay, check for correct charging voltage before continuing a more de· tailed brake system inspection.

PROPORTIONING VALVE A proportioning valve improves brake balance during hard stops by limiting hydraulic pressure to the rear brakes. See Figure 70-9.

A proportioning valve is necessary because inertia creates weight shift toward the front of the vehicle during braking. The weight shift unloads the rear axle, which reduces traction be· tween the tires and the road, and limits the amount of stopping power that can be delivered. Unless application pressure to the rear wheels is limited, the brakes will lock, making the vehicle unstable and likely to spin. The best overall braking performance is achieved when the front brakes lock just before the rear brakes.

Figure 70-9 Many proportioning valves are mounted directly to the master cylinder in the Figure 7(HJ AmagnetiC brake fluid level switch. outlet to the rear brakes.

1100

u; 900 ~ w a:: ::l 700C/) C/) w a:: c. 500 I::l

HYDRAULIC SYSTEM PRESSURETO FRONT BRAKES

\ C. I =::::::::J 1 00 ::l 300 0 /.. 200

SLOPE = ~ =2. = 0.50100 200 2

100 300 500 700 900 1100

INPUT PRESSURE (PSI)

Rgure 70-10 Typical proportioner valve pressure relationship. Note that, at low pressures, the pressure is the same to the rear brakes as is applied to the front brakes. After the split pOint, only apercentage (called the slope) of the master cylinder pressure is applied to the rear brakes.

Vehicles with front disc and rear drum brakes require a proportioning valve for two reasons:

1. Disc brakes require higher hydraulic pressure for a given stop than do drum brakes. In a disc/drum system, the front brakes always need more pressure than the rear brakes.

2. Once braking has begun, drum brakes require less pressure to maintain a fixed level of stopping power than they did to establish that level. In a disc/drum system, the rear brakes will always need less pressure than the front brakes.

A proportioning valve is used to compensate for these differences because it is easier to reduce pressure to the rear brakes than to increase pressure to the front brakes.

The proportioning valve does not work at all times. During light or moderate braking, there is insufficient weight transfer to make rear wheel locking a problem. Before proportioning action will begin, brake system hydraulic pressure must reach a minimum level called the split point. Below the split point full system pressure is supplied to the rear brakes. See Figure 70-10.

Above the split point, the proportioning valve allows only a portion of the pressure through to the rear brakes.

The proportioning valve gets its name from the fact that it regulates pressure to the rear brakes in proportion to the pressure applied to the front brakes. Once system hydraulic pressure exceeds the split point, the rear brakes receive a fixed percentage of any further increase ,in pressure. Brake engineers refer to the ratio of front to rear brake pressure proportioning as the slope. Full system pressure to the rear brakes equals a slope of 1, but if only half the pressure is allowed to reach the rear brakes, the proportioning valve is said to have a slope of 0.50. The proportioning

. valves on most vehicles have a slope between 0.25 and 0.50. See Figures 70-11 and 70-12.

Proportioning Valve Operation A simple proportioning valve consists of a spring-loaded piston that slides in a stepped bore. See Figure 70-13.

The piston is exposed to pressure on both sides. The smaller end of the piston is acted on by pressure from the master cylinder, while the larger end reacts to pressure in the rear brake circuit. The actual proportioning valve is located in the center of the piston and

Chapter 70: Hydraulic Valves and Switches 843

SLOPE/SPLIT POINT RATING

Figure 70-11 AChrysler proportioning valve. Note that slope and split point are stamped on the housing.

Figure 70-12 These two proportioning valves are found under the vehicle on this Dodge minivan.

PISTON TRAVEL BEFORE __~ VALVE CLOSING

PISTON TO REAR BRAKES

SMALL SURFACE AREA

Figure 70-13 The proportioning valve piston can travel within the range shown without reducing pressure to the rear brakes.

is opened or closed depending on the position of the piston in the stepped bore.

When the brakes are first applied, hydraulic pressure passes through the proportioning valve to the rear brakes. Hydraulic pressure is the same on both sides of the piston, but because the side facing the rear brakes has more surface area than the side facing the master cylinder, greater force is developed and the piston moves to the left against the spring tension. At pressures below the split point, the proportioning valve is open, and pressure to both the front and rear brakes is the same.

As the vehicle is braked harder, increased system pressure forces the piston so far to the left that the proportioning valve is closed. See Figure 70-14.

VALVE CLOSED

844 SECTION XI

FROM MASTER CYLINDER VENT

TO REAR BRAKES

Rgure 70-14 At the split paint, the proportioning valve piston closes the fluid passage through the valve.

This seals off the brake line and prevents any additional pres· sure from reaching the rear brakes. The pressure at the moment the proportioning valve first closes is the split pOint of the valve. From this point on, the rear brakes receive only a portion of the pressure supplied to the front brakes.

As system pressure (the pressure to the front brakes) increases, enough force is developed on the master cylinder side of the piston to overcome the pressure trapped in the rear brake circuit. This forces the piston back to the right and opens the proportioning valve. Some of the higher pressure enters the rear brake circuit, but before pressure in the two circuits can equalize, the force devel· oped on the larger piston area in the rear circuit moves the piston back to the left and closes the valve. The difference in surface area between the two ends of the piston determines the slope of the valve, and thus the percentage of system pressure allowed to reach the rear brakes.

As long as system pressure continues to increase, the piston will repeatedly cycle back and forth, opening and closing the pro· portioning valve, and maintaining a fixed proportion of full system pressure to the rear brakes. When the brakes are released, the spring returns the piston all the way to the right, which opens the valve and allows fluid to pass in both directions.

•• :lH: - U:.IIII' I Always Inspect Both Front and Rear Brakes

If a vehicle tends to lock up the rear brakes during a stop, many technicians may try to repair the problem by replacing the proportioning valve or servicing the rear brakes. Proportioning valves are simple spring-loaded devices that are usually trouble free. If the rear brakes lock up during braking, carefully inspect the rear brakes looking for contaminated linings or other problems that can cause the rear brakes to grab. Do not stop there-always inspect the front brakes, too. If the front brakes are rusted or corroded, they cannot operate efficiently and greater force must be exerted by the driver to stop the vehicle. Even if the proportioning valve is functioning correctly, the higher brake pedal pressure by the driver could easily cause the rear brakes to lock up.

A locked wheel has less traction with the road than a rotating wheel. As a result, if the rear wheels become locked, the rear of the vehicle often "comes around" or "fishtails," causing the vehicle to skid. Careful inspection of the entire braking system is required to be assured of asafe vehicle.

HEIGHT-SENSING PROPORTIONING VALVES Many vehicles use a proportioning valve that varies the amount of pressure that can be sent to the rear brakes depending on the height of the rear suspension. This type of valve is called a height-sensing proportioning valve. If the vehicle is lightly loaded, the rear sus· pension is high, especially during braking. In this case, the amount of pressure allowed to the rear brakes is reduced. This helps prevent rear wheel lockup and possible skidding. Besides, a lightly loaded ve· hicle requires less braking force to stop than a heavily loaded vehicle.

When the vehicle is loaded, the rear suspension is forced downward. The lever on the proportioning valve moves and allows a greater pressure to be sent to the rear brakes. See Figures 70-15 and 70-16. This greater pressure allows the rear brakes to achieve more braking force, helping to slow a heavier vehicle. When ave· hicle is heavily loaded in the rear, the chances of rear wheel lockup are reduced.

Some vehicle manufacturers warn that service technicians should never install replacement air lift shock absorbers or springs that may result in a vehide height difference than specified by the vehicle manufacturer.

HEIGHT-SENSING PROPORTIONING VALVE

LINE TO REAR BRAKES

~ .......~ ~

LINE FROM MASTER CYLINDER

Figure 70-15 Aheight'sensing proportioning valve provides the vehicle with variable brake balance. The valve allows higher pressure to be applied to the rear brakes when the vehicle is heavily loaded and less pressure when the vehicle is lightly loaded.

~~~~- PROGRAM SPRING

f igure 70-16 Astepped cam is used to alter the split point of this height-sensing proportioning valve.

Chapter 70: Hydrau lic Valves and Switches 845

PROPORTIONING VALVE ADJUSTMENT Height-sensing proportioning valves should be adjusted when replaced. The proper adjustment ensures that the proper pressure is applied to the rear brakes in relation to the loading of the vehicle.

Procedures vary from one vehicle to another. Always consult the factory service information for the exact procedure. Some trucks require the use of special plastic gauges available from the dealer.

PROPORTIONING VALVE DIAGNOSIS AND TESTING A defective proportioning valve usually allows rear brake pressure to increase too rapidly, causing the rear wheels to lock up during hard braking. When the rear brakes become locked, the traction with the road surface decreases and the vehicle often skids. Whenever rear brakes tend to lock during braking, the proportioning valve should be checked for proper operation. If the proportioning valve is height sensing, verify the proper vehicle ride [trim) height and adjustment of the operating lever. See Figure 70-17_

Pressure gauges can also be used to check for proper operation. Install one gauge into the brake line from the master cylinder and the second gauge to the rear brake outlet of the proportioning valve. While an assistant depresses the brake pedal, observe the two gauges. Both gauges should register an increasing pressure as the brake pedal is depressed until the split point. After the split point, the gauge connected to the proportioning valve (rear brakes) should increase at a slower rate than the reading on the gauge connected to the master cylinder.

If the pressures do not react as described, the proportioning valve should be replaced. The same procedure can be performed on a diagonal split-type system as used on most front-wheel-drive vehicles.

ELECTRONIC BRAKE PROPORTIONING The Delphi DBC-7 eliminates the need for a conventional brake proportioning valve. A proportioning valve is usually necessary to reduce pressure to the rear brakes to keep them from locking up. This is because there is less weight over the rear wheels, and

FROM MASTER CYLINDER (SECONDARY)

INLET FROM MASTER

PRESSURE CYLINDER (PRIMARY)

TO RFTO LF BRAKE BRAKE

II II II

:~ OUTLET PRESSURE

:: TO RR BRAKE II

weight shifts fOf\vard when braking. Proportioning is needed most when a vehicle is lightly loaded or braking from a high speed. Most proportioning valves are calibrated to reduce pressure to the rear brakes by a fixed amount, which may increase the risk of rearwheel lockup if the vehicle is loaded differently or is braking on a wet or slick surface. Dynamic rear proportioning is overcome by adjusting brake balance to match the need of the vehicle to changing road and load conditions.

Electronic brake proportioning in the DBC-7 system is accomplished by monitoring front- and rear-wheel speeds, and reducing pressure to the rear brakes as needed using the ABS solenoids when there is a difference in wheel deceleration rates. The controller energizes the inlet valve solenoids for both rear brakes to hold pressure in the lines, and then energizes both rear outlet valve solenoids to release pressure as needed. The pump may also run to clear the accumulators if a sufficient number of release cycles are required .

The dynamic rear proportioning function is enabled at all times unless there is a failure of the EBCM or two wheel speed sensors on the same axle both fail simultaneously. But as long as there is at least one functional speed sensor on the front and rear axles, the ElectroniC Brake Control Module (EBCM) can compare the relative speeds of the front and rear wheels.

METERING VALVE (HOLD-OFF) OPERATION A metering valve is used on all front-disc, rear-drum-brakeequipped vehicles. The metering valve prevents the full operation of (holds off) the disc brakes until between 75 and 125 PSI is sent to the rear drum brakes to overcome rear-brake return spring pressure. This allows the front and rear brakes to apply at the same time for even stopping. Most metering valves also allow for the pressure to the front brakes to be gradually blended up to the metering valve pressure to prevent front brake locking under light pedal pressures on icy surfaces.

Ametering valve consists of a piston controlled by a strong spring and a valve stem controlled by a weak spring. See Figure 70-18.

FROM MASTER CYLINDER (PRIMARY)

INLETFROM MASTER PRESSURECYLINDER

(SECONDARY)

TO LF TO RF BRAKEBRAKE

II II

~: TO LR BRAKE : :

OUTLET PRESSURE

Figure 70-17 Aproportioning valve pressure test can be performed using two pressure gauges-one to register the pressure from the master cylinder and the other gauge to read the pressure being applied to the rear brakes. This test has to be repeated in order to read the pressure to each rear wheel.

846 SECTION XI

VALVE STEM

COMPRESSED SPRING

VALVE OPEN

STEM

COMPRESSED 3TO 30 PSI SPRING PRESSURE

Figure 7(}-19 A metering valve under light brake pedal application.

When the brakes are not applied, the strong spring seats the piston and prevents fluid flow around it. At the same time, the weak spring holds the valve stem to the right and opens a passage through the center of the piston. Brake fluid is free to flow through this passage to compensate for changes in system fluid volume.

When the brakes are applied and pressure in the front brake line reaches 3 to 30 PSI (20 to 200 kPa ), the tension of the weak spring is overcome and the metering valve stem moves to the left, which closes the passage through the piston and prevents fluid flow to the front brakes. See Figure 70-19.

The small amount of pressure applied to the calipers before the metering valve closes is enough to take up any clearance, but not enough to generate braking force.

While the fluid flow to the front calipers is shut off, the rear brake shoes move into contact with the drums, braking begins, and hydraulic pressure throughout the brake system increases. When the pressure at the metering valve reaches 75 to 300 PSI, the tension of the strong spring is overcome and the valve stem and piston move farther to the left. See Figure 70-20.

This opens a passage around the outside of the piston and allows fluid to flow through the valve to the front brake calipers.

NOTE: Braking systems that are diagonal split, such as those found on most front-wheel-drive vehicles, do not use a metering valve. A metering valve is only used on front/rear split braking systems such as those found on most rear-wheel-drive vehicles.

SYSTEMS WITHOUT METERING VALVES There are three reasons front-wheel-drive vehicles do not use metering valves:

1. Front-wheel-drive vehicles usually have a diagonally split dual braking system that would require a separate metering valve for each hydraulic circuit. This would make the brake system more costly and complicated.

2. Front-wheel-drive vehicles have a forward weight bias that requires the front brakes to supply up to 80% of the total braking power. Since the front brakes do most of the work, it is desirable to apply them as soon as possible when the brake pedal is depressed. Ametering valve would create a slight delay.

3. Until all the clearance in the brake system is taken up, there will not be enough pressure in the brake hydraulic system for the front disc brakes to overcome the engine torque applied to the driven front wheels.

Engine torque and a heavy front weight bias help prevent front wheel lockup from being a: problem during light braking or when the brakes are first applied.

PISTON

VALVE STEM

FROM ~MASTER

~~~~~ CYLINDER

WEAK VALVE

SPRING OPEN

TO FRONT BRAKE

Figure 7(}-18 A metering valve when the brakes are not applied. Notice the brake fluid can flow through the metering valve to compensate for brake fluid expansion and contraction that occurs with changes in temperature.

PISTON VALVE CLOSED VALVE

Figure 70-20 A metering valve during a normal brake application.

When the brakes are released, the strong spring seats the piston and prevents fluid flow around it. At the same time, the weak spring opens the fluid passage through the center of the piston. Excess fluid returns to the master cylinder through this passage and the valve is ready for another brake application.

nI:fH: IlII:.l V I No Valves Can Cause a Pull

When diagnosing a pull to one side during braking, some technicians tend to blame the metering valve, proportional valve, the pressure differential switch, or the master cylinder itself.

Just remember that if a vehicle pulls during braking that the problem has to be due to an individual wheel brake or brake line.The master cylinder and all the valves control front or rear brakes together or diagonal brakes and cannot cause a pull if not functioning correctly.

Most rear-wheel-drive vehicles without metering valves are equipped with four-wheel disc brakes. Because the clearance between the pads and rotors is approximately the same at all four wheels, there is no need to delay front brake actuation. Some of these vehicles also have antilock brake systems that prevent the wheels from locking at any time. Other rear-wheel-drive vehicles without metering valves have a predominantly forward weight bias, like front-wheel-drive vehicles, and therefore benefit from having the front brakes applied sooner.

METERING VALVE DIAGNOSIS AND TESTING A defective metering valve can leak brake fluid and/ or cause the front brakes to apply before the rear brakes. This is most commonly noticed on slippery surfaces such as on snow or ice or on rain-slick roads. If the front brakes lock up during these conditions, the front wheels cannot be steered. Inspect the metering for these two conditions:

1. Look around the bottom of the metering valve for brake fluid leakage. (Ignore slight dampness.) Replace the metering valve assembly if it is leaking.

2. As the pressure builds to the front brakes, the metering valve stem should move. If it does not, replace the valve.

More accurate testing of the metering valve can be accomplished using pressure gauges. Install two gauges, one in the pressure line coming from the master cylinder and the other in the outlet line leading to the front brakes. When depressing the brake pedal, both gauges should read the same until about 3 to 30 PSI (20 to 200 kPa) when the metering valve shuts, thereby delaying the operation of the front brakes. The master cylinder outlet gauge should show an increase in pressure as the brake pedal is depressed further.

Once 75 to 300 PSI is reached, the gauge showing pressure to the front brakes should match the pressure from the master cylinder. If the pressures do not match these ranges, the metering valve assembly should be replaced .

Chapter 70:HydraulicValves and SWITches 847

SWITCH TERMINAL

PROPORTIONING-PRESSURE DIFFERENTIAL

METERING-PRESSURE DIFFERENTIAL

Figure 70-21 Typical two-function combination valves.

COMBINATION VALVE Most vehicle manufacturers combine the function of a proportioning valve with one or more other valves into one unit called a combination valve. See Figures 70-21 and 70-22.

On a typical rear-wheel-drive vehicle, a typical combination valve consists of the following components all in one replaceable unit:

• Metering valve • Proportioning valve • Pressure-differential switch

Some combination valves have only two functions and contain the pressure-differential and the metering valve, while others combine the pressure-differential with the proportioning valve.

BRAKE LIGHT SWITCH The job of the brake light switch is to turn on the brake lights at the back of the vehicle when the brakes are applied. A properly ad· justed light switch will activate the brake lights as soon as the brake pedal is applied and before braking action actually begins at the wheels.

Mechanical switches that operate directly off the brake pedal arm are most often used. See Figure 70-23.

Brake light switches are normally open. When the brakes are applied, the switch closes, which completes the brake light circuit.

Always check service information for the specified procedures to follow when replac ing and/or adjusting a brake switch to ensure proper operation.

OTE: Neither the metering valve nor the proportioning valve can cause a pull to one side if defective. The metering valve controls both front brakes, and the proportioning valve controls both rear brakes. A defective master cylinder cannot cause a pull either. Therefore, if a vehicle pulls to one side during a stop, look for problems in the individual wheel brakes, hoses, or suspension.

•• :{Ii:•• 1~.tI Push-In or Pull-Out Metering Valve?

Whenever bleeding the air out of the hydraulic brake system,the metering valve should be bypassed. The metering valve stops the passage of brake fluid to the front wheels until pressure exceeds about 125 PSI (860 kPa). It is important not to push the brake pedal down with a great force so as to keep from dispersing any trapped air into small and hard-to-bleed bubbles. To bypass the metering valve, the service technician has to push or pull a small button located on the metering valve. An easy way to remember whether to push in or to pull out is to inspect the button itself. If the button is rubber coated, then you push in. If the button is steel, then pull out.

Special tools allow the metering valve to be held in the bypass position. Failure to remove the tool after bleeding the brakes can result in premature application of the front brakes before the rear drum brakes have enough pressure to operate.

848 SECTION XI

METERING VALVE

PRESSURE DIFFERENTIAL SWITCH

PROPORTIONING VALVE

OUTLET TO FRONT BRAKE

INLET FROM MASTER CYLINDER

Figure 7D-22 Combination valve containing metering, pressure-differential (warn ing switch), and proportioning valves all in one unit. This style is often called a "pistol grip" design because the proportioning valve section resembles the grip section of a hand gun.

Figure 7D-23 Typical brake light switches.

SUMMARY 1. Residual check valves are used in older vehicles to keep a slight amount

of pressure on the system to help prevent air from entering the system when the brake pedal is released.

2. A pressure·differential switch is used to turn on the red brake warning lamp in the event of a hyd ra ulic pressure failure.

3. Brake fluid level sensors are used in many vehicles to warn the driver that the brake fluid level is low.

4. Proportioning valves are used to limit the maximum fluid pressure sent to the rear wheel brakes during heavy braking to help prevent rear wheel lockup.

5. Metering valves are used on some vehicles to keep the front disc brakes from locking up on slippery surfaces.

6. Combination valves include two or more hydraulic valves in one assembly.

REVIEW QUESTIONS 1. Why are residual check valves not used in most vehicles?

2. List the Ulree possible reasons that could cause the red brake warning lamp to come on during driving.

3. Explain why metering valves are not used on all vehicles.

4. Explain the split point and the slope of a proportioning valve.

CHAPTER QUIZ 1. Technician A says a pull to the right during braking could be caused by

a defective metering valve. Technician B says a pull to the left could be caused by a defective proportioning valve. Which technician is correct?

a. Technician A only b. Technician Bonly c. Both Technicians A and B d. Neither Technician A nor B

2. The rear brakes lock up during a regular brake application. Technician A says the metering valve could be the cause. Technician B says that stuck front disc brake calipers could be the cause. Which technician is correct?

a. Technician A only b. Technician B only c. Both Technicians A and B d. Neither Technician A nor B

3. The rear wheels lock up during hard braking. Technician A says that a defective metering valve could be the cause. Technician Bsays that a defec· tive proportioning valve could be the cause. Which technician is correct?


4. A combination valve includes _____ a. Metering and proportioning valves b. Proportioning and pressure·differential valves c. Proportioning, metering, and pressure·differential valves d. Any of the above depending on the make and model of the vehicle

5. A residual check valve is used to _____ a. Maintain a slight pressure on the hydraulic system b. Prevent front wheel lockup during hard braking c. Prevent rear wheel lockup during hard braking d. Speed brake release to reduce brake wear

6. Technician Asays that Ule red brake warning light can be turned on if a differ· ence in pressure is detected by the pressure·differential switch. Technician B says that the red brake warning light can be turned on if the brake fl uid level sensor detected low brake fluid level. Which technician is correct?

a. Techn ician A only b. Technician Bonly c. Both Technicians A and B d. Neither Technician A nor B

7. Which type of vehicles most often do not use metering valves? a. Rear-wheel drive b. Four-wheel drive c. Front-wheel drive d. All-wheel drive

8. A spongy brake pedal is being diagnosed. Technician A says that air in the hydraulic system could be the cause. Technician Bsays a defective pressure· differential switch could be the cause. Which technician is correct?


9. The button on the valve should be held when pressure bleeding the brakes.

a. Metering b. Proportioning c. Pressure-differential d. Residual check

10. A typical brake light is electrically _____ a. Normally open b. Normally closed

Chpt 70 Hydraulic Valves and Switches

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