A common paper clip or piece of stiff wire can simplify internal leak diagnosis on some light-truck ABS systems. Also, a special bleeding procedure may remove the dirt causing this internal leak, according to service specialists at Wagner Brake Products.
Leaking EH actuator
An ABS system prevents wheel lockup during panic stops by regulating brake application pressure very rapidly and precisely. The ABS computer does this by operating one or more valves inside the electrical/
hydraulic (EH) actuator.
Since about 1987, the Kelsey Hayes rear-wheel ABS system has appeared on many domestic light trucks, including those built by Chrysler Corp., Ford Motor Co., and General Motors Corp. These ABS systems may benamed differently, but all work the same and share a common EH actuator assembly.
The EH actuator is located below the master cylinder on GM applications and on the frame of Fords and Chryslers.
A common failure on Kelsey Hayes rear-wheel ABS is an internal EH actuator leak, which causes a low, spongy or sinking brake pedal complaint. According to every brake expert interviewed, dirty brake fluid is the root cause of internal EH actuator leaks. Dirt and rust particles in neglected brake fluid prevent the dump valve from sealing off the accumulator chamber.
In Figure 1, an illustration of the earlier-design, cast iron EH actuator, the accumulator is on the left. In Figure 2, the later-design aluminum EH actuator, the accumulator is on the right.
One way the Kelsey Hayes EH actuator modulates brake pressure during a panic stop is by opening the dump valve. This reduces rear-brake application pressure by venting some of that pressure into the accumulator chamber, which in turn pushes the accumulator piston outward.
Remember that the dump valve is normally closed, sealing off the accumulator port from the rear brake circuit. Because the dump valve should open only during an ABS-assisted stop, the accumulator piston should only be in the outward position during ABS operation-not during normal braking.
When the vehicle has a low, spongy brake pedal and the EH actuator accumulator piston has moved outward during normal brake operation, techs can conclude the dump valve is leaking, said Ned Lineback, technical instructor at Wagner Brake Products.
EH actuator check
Technicians often mistake a leaking dump valve for a failing master cylinder because the two conditions cause similar symptoms. Working methodically is essential to isolating a suspect EH actuator from a failing master cylinder or a hydraulic leak in the rear brake circuit, brake experts said.
Mr. Lineback recommended using a length of stiff wire or a straightened paper clip to determine if the accumulator piston is sliding outward during normal brake application. To do this, remove the end cap from the EH actuator assembly and drill an access hole just large enough to accommodate the paper clip or stiff wire. Drill the access hole toward the center of the end cap so the paper clip or wire won't encounter interference from the accumulator return spring.
Note that the end cap on the later-model EH actuator already has a hole drilled in it.
Reinstall the end cap on the EH actuator and slide the paper clip or stiff wire through the access hole until it bottoms against the accumulator piston. Next, watch the paper clip or wire while an assistant starts the engine and depresses the brake pedal.
If the dump valve is closed, the paper clip or wire won't move when the brakes are applied. But if the accumulator piston pushes the paper clip outward when the assistant applies the brakes, it confirms the dump valve is leaking, Mr. Lineback said.
Ordinarily, a leaking dump valve requires replacing the EH actuator. But Mr. Lineback said some techs manage to flush the dirt out of the dump valve with a power bleeder. To do this, top off the master cylinder with fresh brake fluid and connect a power bleeder to the master cylinder. Open a rear bleeder fitting, connect a drain hose to the fitting and drape the hose into a suitable container.
Then trick the ABS system into performing its self test by cycling the ignition switch on and off repeatedly. The ABS computer operates the dump valve once per self test to be sure the valve works. When the dump valve opens, the power bleeder will push fluid through it. The fluid rushing through the dump valve may sweep away dirt that's preventing the valve from sealing itself against the accumulator port.
Close the rear bleeder fitting and repeat the paper clip movement test. Replace the EH actuator if the paper clip still moves outward when the brakes are applied.
If the EH actuator now passes the paper clip check, remove the paper clip and reseal the hole with duct tape, a dab of RTV silicone sealer or other suitable sealer, Mr. Lineback said.
When the symptom is a low, spongy brake pedal and the paper clip test shows the dump valve is not leaking, you must still isolate the master cylinder from the rear brake circuit. To do this, disconnect the brake line serving the rear brake circuit from the master cylinder. Install the appropriate blank plug from a master cylinder bleeding kit into this port, then bleed the master cylinder.
Mr. Lineback said techs can save time by having an assistant depress the brake pedal about an inch to push air out of the rear-circuit master cylinder port. Have the assistant maintain this brake pedal position while you install and tighten the solid plug.
After you tighten the plug, instruct your assistant to continue depressing the brake pedal. This prevents the primary master cylinder cup from being damaged as it moves across the vent port, he said.
If brake pedal height and firmness are normal with the solid plug installed, it tells you the master cylinder is OK. Check for a leak in the rear hydraulic circuit. But if the pedal is still low and spongy, the master cylinder is failing and must be replaced. Regardless of which repair is required, always flush the brake system thoroughly with clean brake fluid.