Problem: Low Brake Pedal
Bypassing dump valve in modulator allows fluid into the low-pressure accumulator causing low brake pedal on vehicles equipped with RWAL or RABS ABS systems.Low Brake Pedal Solution
Whenever an RWAL/RABS equipped vehicle presents itself with a low brake pedal, the first thing that you should check for is a bypassing dump valve. The method of testing depends on the style of the valve. These vehicles use two different design modulators.The steps used in the diagnosis of the dump valve will depend on the modulator design. The first step is to locate the modulator and determine its type. Then use the following information to diagnose it. (Note: The “Type” designations are assigned by me for reference in this article and are not Kelsey Hayes designations)
Type 1: Rear Wheel ABS "Torpedo Style"
This valve is one of 2 designs used on vehicles equipped with rear-wheel ABS. It is identified by its cylindrical shape, as shown in Figure 44.1. To perform the dump valve diagnosis, follow the steps below1. Remove the 1-1/4” cap screw at the end of the valve body as well as the low-pressure accumulator spring.
2. Insert a screwdriver or similar tool into the backside of the low-pressure accumulator piston. (See images below)
3. Have someone start the vehicle and apply the brakes while slowly increasing brake pedal pressure.
NOTE: The brake pedal fade must be duplicated for the test to be accurate.
4. If the low-pressure accumulator piston pushes against the screwdriver, the dump valve bypasses, and you will have to replace the modulator.
Type 2: Rear Wheel ABS "Block Style"
Block style is the other valve used on light trucks equipped with rear-wheel ABS. It is identified by its block-shaped body, as shown below. To perform the dump valve diagnosis, follow the steps below:1. Locate the low-pressure accumulator cap and remove the rubber cap covering the cap vent.
2. Insert a straightened paper clip into the cap screw vent hole until it bottoms out, as shown below.
3. Have someone start the vehicle and apply the brakes while slowly increasing brake pedal pressure.
NOTE: The brake pedal fade must be duplicated for the test to be accurate.
4. If the paper clip pushes out, the dump valve bypasses, and you will have to replace the modulator.
Confirming Low Brake Pedal Diagnosis
If the above steps indicate a bypassing dump valve, you will need to replace the modulator. However, before installing the modulator, you should check the system for additional causes of excessive pedal travel. The most effective method to accomplish this is to temporarily “fix” the modulator. Doing so can be achieved by following the steps below.Confirmation on RABS/RAWL System
1. Remove the cap screw and low-pressure accumulator spring.2. Insert a solid spacer in place of the spring to prevent the low-pressure accumulator piston from moving. Install the cap screw. (See Figure 44.7).
3. Start the vehicle and check the brake pedal. If the pedal feel is now correct, all that is necessary is to complete modulator replacement. If the pedal is better but still low, perform a line lock test to determine the cause of additional pedal travel.
NOTE: You should use spacers as a diagnostic technique only. You should never leave them in place.
Intermittent Diagnosis - Low Brake Pedal
Dump valve failure can occur on an intermittent basis making a diagnosis in the shop difficult to impossible. If the failure is intermittent and can only duplicate it on a road test, use the steps below. There are two designations to simplify the diagnosis.Vented Cap Screw Design (Rear Wheel ABS "Block Style")
1. Insert the straightened paper clip through the rubber vent cap as shown below until it bottoms out.2. Use a magic marker or piece of tape to mark the position of the paper clip relative to the rubber cap, as shown below.
3. Test drive the vehicle until you can duplicate the pedal fade.
4. Check the position of the paper clip. If the paperclip falls out, dump valve failure has occurred, and the modulator requires replacement (See Image Below). On the other hand, if no movement has taken place, the source of the pedal fade is not ABS-related.
NOTE: It is crucial not to engage the ABS during this test. Low-pressure accumulator movement occurs during the ABS cycle.
Non-Vented Cap Screws (Torpedo Style Rear Wheel ABS)
1. Remove the cap screw and low-pressure accumulator spring. Note the piston position. Reinstall the cap screw.2. Test drive vehicle until you duplicate the pedal fade.
3. Remove cap screw and check low-pressure accumulator position. If the piston is against the cap screw, dump valve failure has occurred, and the modulator you will have to replace the modulator
NOTE: it is vital not to engage the ABS during this test. Low-pressure accumulator movement occurs during the ABS cycle.
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