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A residual check valve has been used on some drum brake systems to keep a slight amount of pressure on the entire hydraulic system for drum brakes (5 to 12 psi).
Continued Figure 70–1 Most residual check valves are located under the tubing seals in the master cylinder outlet ports. The check valve is located in the master cylinder at the outlet for the drum brakes. The ball and spring in the 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.
Figure 70–2 The momentary drop in pressure created when the brakes are released can draw air into the hydraulic system. Continued
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.
Figure 70–3 The use of cup expanders is the main reason why residual check valves are not used in most braking systems today.
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.
PRESSURE-DIFFERENTIAL SWITCH (BRAKE WARNING SWITCH) Figure 70–4 A red brake warning lamp. Continued The brake lines from both front and rear sections of the master cylinder are sent to this switch It lights the brake warning indicator lamp in the event of a “difference in pressure.” See Figure 70–5.
Figure 70–5 A leak 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. (Courtesy of Ford Motor Company)
If the lamp remains on:
Apply light pressure to the brake pedal.
Momentarily open the bleeder valve on the side that did not fail.
Figure 70–6 The pressure-differential switch piston is used to provide the electrical ground for the red brake warning light circuit.
Those procedures should center the pressure-differential switch valve in those vehicles that are not equipped with self-centering springs.
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.
Figure 70–7 A movable contact brake fluid level switch. Figure 70–8 A magnetic 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.
Activation of the red brake dash warning lamp can be for any one of several reasons:
DIAGNOSING A RED “BRAKE” DASH WARNING LAMP Continued 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 vehicles 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.
The likely cause of the red “brake” warning lamp is low brake fluid caused by a leaking brake line, wheel cylinder, or caliper. The first step is to determine the cause of the lamp being on, then to repair the problem.
Continued Step #1 Check the Level of the Brake Fluid If low, carefully inspect entire hydraulic brake system for leaks; repair as necessary. Step #2 Disconnect the Wire from the Pressure - Differential Switch If the lamp is still “on,” the problem is the parking brake lever switch, or the wire going to the switch, shorted to ground. If the red brake warning lamp is “off” after being disconnected from the pressure-differential switch, the problem is due to a hydraulic failure (a low pressure in either front or rear system creating a pressure difference 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 detailed brake system inspection.
A proportioning valve improves brake balance during hard stops by limiting hydraulic pressure to the rear brakes. The valve is necessary because inertia creates weight shift toward the front of the vehicle during braking. The shift unloads the rear axle, which reduces traction between 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.
See Figure 70–9.
Figure 70–9 Many proportioning valves are mounted directly to the master cylinder in the outlet to the rear brakes. Continued
Vehicles with front disc and rear drum brakes require a proportioning valve for two reasons:
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.
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.
Figure 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 a percentage (called the slope) of the master cylinder pressure is applied to the rear brakes.
The proportioning valve does not work at all times.
During light or moderate braking, there is no rear wheel locking a problem. Before proportioning will begin, hydraulic pressure must reach a level called the split point . Continued
Figure 70–11 A Chrysler proportioning valve. Note that slope and split point are stamped on the housing.
Above the split point, the proportioning valve allows only a portion of the pressure through to the rear brakes.
Figure 70–12 These two proportioning valves are found under the vehicle on this Dodge minivan. Below the split point full system pressure is supplied to the rear brakes. Continued
Proportioning Valve Operation A simple proportioning valve consists of a spring-loaded piston that slides in a stepped bore. 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 is opened or closed depending on the position of the piston in the stepped bore.
See Figure 70–13.
Figure 70–13 The proportioning valve piston can travel within the range shown without reducing pressure to the rear brakes. Continued
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.
Figure 70–14 At the split point, the proportioning valve piston closes the fluid passage through the valve. Continued
If a vehicle tends to lock up the rear brakes during a stop, many techs 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.
Always Inspect Both Front and Rear Brakes A locked wheel has less traction with the road than a rotating wheel. As 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 a safe vehicle. 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.
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 . 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. This greater pressure allows the rear brakes to achieve more braking force, helping to slow a heavier vehicle. When heavily loaded in the rear, chances of rear wheel lockup are reduced.
See Figures 70–15 and 70–16.
Figure 70–15 A height-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. Continued
Figure 70–16 A stepped cam is used to alter the split point of this height-sensing proportioning valve. CAUTION: Some vehicle manufacturers warn that service techs should never install replacement air lift shock absorbers or springs that result in a vehicle height difference than specified by the manufacturer.
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.
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. 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. See Figure 70–17.
PROPORTIONING VALVE DIAGNOSIS AND TESTING Continued
Figure 70–17 A proportioning 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. Continued
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. 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 weight shifts forward when braking. Proportioning is needed most when a vehicle is lightly loaded or braking from a high speed. Most valves are calibrated to reduce pressure to the rear brakes by a fixed amount, which may increase the risk of rear-wheel lockup if the vehicle is loaded differently or is braking on a wet or slick surface. Dynamic rear proportioning is overcome by adjusting balance to match 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 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. 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-brake-equipped vehicles. The 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 front and rear brakes to apply at the same time for even stopping. Most metering valves also allow for 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. A metering valve consists of a piston controlled by a strong spring and a valve stem controlled by a weak spring. See Figure 70–18.
Figure 70–18 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. Continued
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. 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.
Figure 70–19 A metering valve under light brake pedal application. Continued
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, 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, tension of the strong spring is overcome and the valve stem and piston move farther to the left. This opens a passage around the outside of the piston and allows fluid to flow through the valve to the front brake calipers. See Figure 70–20.
Figure 70–20 A metering valve during a normal brake application. Continued
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.
No Valves Can Cause a Pull 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. NOTE: 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.
SYSTEMS WITHOUT METERING VALVES
There are three reasons front-wheel-drive vehicles do not use metering valves:
Front-wheel-drive vehicles usually have a more costly and complicated diagonally split dual braking system that requires a separate metering valve for each hydraulic circuit.
Front-wheel-drive vehicles have a forward weight bias that requires front brakes to supply up to 80% of the total braking power. A metering valve would create a delay.
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. 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. Most noticed on slippery surfaces such as on snow or ice or on rain-slick roads.
Continued If the front brakes lock up during these conditions, the front wheels cannot be steered. Inspect the metering valve for these conditions:
Look around the bottom of the metering valve for brake fluid leakage. (Ignore slight dampness.) Replace the metering valve assembly if it is leaking.
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 from the master cylinder, the other in the outlet line leading to the front brakes.
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. When pressing the brake pedal, both gauges should read the same until 3 to 30 psi (20 to 200 kPa) when the metering valve shuts, delaying operation of the front brakes. The master cylinder gauge should show an increase in as the brake pedal is pressed 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.
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.
Push - In or Pull - Out Metering Valve? 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.
Most manufacturers combine the function of a proportioning valve with one or more other valves into one unit called a combination valve .
Continued Figure 70–2 Typical two-function combination valves.
On a rear-wheel-drive vehicle, a typical combination valve consists of these components, all in one replaceable unit:
Some valves have two functions and contain pressure-differential and metering valve; others combine the pressure-differential with the proportioning valve.
Figure 70–22 Combination valve containing metering, pressure-differential (warning switch), and proportioning valves all in one unit. This style is often called a “pistol grip” design because the valve section resembles the grip section of a hand gun. Continued
The brake light switch turns on brake lights at the back of the vehicle when brakes are applied. A properly adjusted light switch will activate brake lights as soon as the pedal is applied and before braking action actually begins at the wheels.
BRAKE LIGHT SWITCH Figure 70–23 Typical brake light switches. CAUTION: Check service information for procedures to follow when replacing and/or adjusting a brake switch to ensure proper operation. Mechanical switches operating directly off the brake pedal arm are most often used. Brake light switches are normally open; when brakes apply, the switch closes, completing the light circuit.
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.
A pressure-differential switch is used to turn on the red brake warning lamp in the event of a hydraulic pressure failure.
Brake fluid level sensors are used in many vehicles to warn the driver that the brake fluid level is low.
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.
Metering valves are used on some vehicles to keep the front disc brakes from locking up on slippery surfaces.
Combination valves include two or more hydraulic valves in one assembly.