A leaking electrical/hydraulic (EH) actuator on some rear-wheel ABS systems may be mistaken for a bad master cylinder, brake experts said. As described in a previous service supplement (August 9, 1993), an ABS computer prevents wheel lockup during sudden stops by modulating brake application pressures. It does this by activating the EH actuator or control valve.
Since the late 1980s, many popular Ford Motor Co., Chrysler Corp. and General Motors Corp. light trucks have been equipped with Kelsey Hayes rear-wheel ABS. These systems may have different names (RABS, RWAL etc.), but all work the same way and use the same EH actuator.
Note that the EH actuator is mounted directly below the master cylinder on GM trucks, but on the frame of Fords and Chryslers.
We explained in the August, 1993 service supplement that neglected brake fluid is taking its toll on ABS hydraulic components. The single most common example sources cited is the dump valve inside the non-serviceable Kelsey-Hayes EH actuator on light trucks.
Dirt and corrosion in the brake fluid can prevent the dump valve from reseating itself against the port of an accumulator inside the EH actuator, said Joe Meyer, an instructor at Professional Automotive Career Training (PACT), a division of Conrad's Goodyear, Cleveland, Ohio.
When the driver applies the brakes, some brake fluid bleeds back into the accumulator instead of filling the rear wheel cylinders and applying the rear brakes. This causes the brake pedal to drop abnormally low and to feel soft or spongy, though sometimes the driver can temporarily improve pedal firmness by pumping the pedal quickly.
These symptoms suggest a bad master cylinder or other hydraulic problem, but replacing the master cylinder may not solve the problem.
There are three potential causes of the low, spongy brake pedal condition described above-a bad master cylinder, leaking dump valve inside the EH actuator or a hydraulic leak in the rear brake circuit.
Sources agreed the easiest way to pinpoint the problem is to isolate one part of the brake system at a time. To do this, technicians need plug fittings from a master cylinder bench bleeding kit.
Begin by disconnecting the steel brake lines from the master cylinder and installing the proper threaded plugs in their place. Then slowly bleed the master cylinder at these outlet ports. The master cylinder is good if this procedure yields a firm, high brake pedal. If not, the master cylinder must be replaced.
If the master cylinder is OK, connect the steel lines to it again and bleed it at the lines. Next, locate the EH actuator assembly and disconnect the steel line going from the actuator to the rear brake circuit. Install the proper threaded plug into the EH actuator outlet port, bleed the actuator and recheck brake pedal firmness.
If the brake pedal is low and spongy, replace the EH actuator because its dump valve is leaking internally. But if the pedal is now firm and high, you know by process of elimination that the hydraulic leak is in the rear brake circuit.
Check for leaking rear wheel cylinders, a leaking rear brake hose or a pinhole corroded through a steel brake line.
Remember that pinhole leaks are easier to see when an assistant pressurizes the line by applying the brake pedal. Always wear appropriate eye protection when searching for brake fluid leaks under the vehicle.
According to some brake specialists, disconnecting the brake line from the EH actuator's inlet port can give a valuable clue when diagnosing a leaking dump valve. When the fluid is so dirty that it's causing a dump valve leak, dirt and scale will be visible in the filter screen inside the actuator inlet port.