While servicing ABS-equipped vehicles is much the same for conventional pad and rotor replacement, the number of additional components in such systems can sometimes lead to peculiar problems.
One such circumstance is excessive front brake wear on vehicles equipped with the Kelsey Hayes Rear Wheel Anti-Lock system (mostly light trucks and vans from 1987 to 2000).
This condition can be present at almost any stage of the vehicle’s life, but appears more often after the 80,000-km mark. Contaminated brake fluid, excessive brake pedal travel, or a mushy or spongy brake pedal may also accompany the severely shortened pad life. Faced with these symptoms, the initial reaction by a technician might be to consider replacing the master cylinder. However, he should investigate a possible problem with the electric hydraulic anti-lock control. This device has a unique solenoid valve; while most anti-lock units have normally open valves, this unit has a normally closed valve. When the valve opens during an anti-lock stop, any contamination in the brake fluid can interfere with the valve seating properly. There is no fault registered when this happens, but the non-seating valve allows brake fluid to enter the accumulator when the brake pedal is depressed. This extra expansion doesn’t allow enough pressure to build in the rear brake circuit, so the rear brakes don’t apply properly. This transfers an excessive amount of brake work to the front brakes, causing them to wear much faster than they were intended.
A lack of pressure can be detected if a set of pressure gauges is hooked up to the front caliper and rear wheel cylinder. The master cylinder still needs to be isolated from the circuit to determine if the cause is, in fact, a bad seal in the master cylinder.
If you have a technician facing such a situation, advise him to isolate the problem by undoing the rear brake line where it enters the anti-lock hydraulic unit. He can cap the line and pump up the brakes with the brake pedal in the vehicle. He should be able to achieve a high, hard pedal. If he cannot, it is a good indicator that the seals in the master cylinder are bypassing and in need of service.
Of course, any other leaks in the system also have to be ruled out. But if the brake pedal pumps up hard, the technician can reinstall the brake line into the anti-lock unit and proceed to disconnect the brake line coming out of the hydraulic unit and plug the hydraulic unit.
When he attempts to pump up the brake pedal now, he should experience more travel and feel a mushier pedal. This indicates that the dump valve is allowing brake fluid to enter the accumulator and warrants a new hydraulic device.
It is not advisable to attempt to repair or flush the unit, but to replace it with a new or remanufactured unit.
While it may seem a shame to have to replace such a unit for a few grains of dirt, it is useful to remember that it is part of a critical safety system and that chances should not be taken with performance or reliability.
Information for the above was provided by American Remanufacturers Inc.