Brakeless, Thanks for your question on Just Answer. There is a pretty common problem that is over looked or forgotten about. Check the Dump valve, mounted on the frame on drivers floor board. I am attaching the TSB that deals with this. To test and verify that this is the problem you can use a piece of brake line and bypass the unit. Good Luck and if I can be of further assistance please let me know. Keith
WAGNER BRAKE CENTER
Bulletin No. 24
Date November, 1989
DODGE FORD and GENERAL MOTORS TRUCKS 87-90 BRONCO BRONCO II/RANGER F,E Equipped with Rear Anti-Lock Brake System
|Ford:||RABS (Rear Anti-Lock Brake System) GM & Dodge: RWAL (Rear Wheel Anti-Lock) Topic |
False or Dropping Pedal
Some of the above-mentioned vehicles may experience a "false" or dropping pedal condition that may feel very similer to a master cylinder with an excessive amount of by-pass. Upon installation of the replacement master cylinder, there is little or no improvement in pedal feel or height. This condition may be caused by the dump valve in the E-H (Electro Hydraulic) valve being held slightly open by a small particle of foreign material. This allows fluid to flow into the accumulator and create the low or dropping brake pedal.
To determine if this is the cause of the dropping pedal, begin by disconnecting the steal line at the master cylinder that serves the rear brakes. Install a solid tubing plug into the outlet that serves the rear brakes. Note: When installing the plug, have an assistant depress the pedal about one inch to purge any air from this outlet. Tighten the plug while the assistant holds the pedal in this position. After, tightening the plug, continue to apply pressure to the pedal. This will prevent damage to the primary cup in the master cylinder as the cup moves across the vent port. If the pedal no longer drops as it did with the rear brakes connected, this would indicate that the problem is in the rear brakes and not the master cylinder.
The next step would be to block off the outlet port of the E-H valve and retest the brake pedal as done previously. If the dropping pedal reappears, the problem is in the E-H valve and most likely caused by fluid leaking into the accumulator of the E-H valve. If this is the case, it will be necessary to change the valve. The E-H valve, including the accumulator and valving, is not serviceable.
Note that brake fluid over a period of time absorbs contaminants, therefore, annual flushing and replacement with clean, fresh brake fluid will help to prevent this condition from occurring.