I have a 1987 f150 4x4 4-sp manual with a 5.0liter. last month the brake pedal goes to the floor when pressed, only somne times, but it is unpredictable. I have tried 3 new master cylinders, a new prtoportioning valve.
and a new vacumn assist. nothing seems to work to eliminate the problem. the truck does have ABS rears, not the electronic type. PLEASE HELP!!!!

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mash

 

Where are you buying the master cylinders from? I bought a master cylinder for my old nissan at PepBoys once. I changed it out three times under pepboys warranty in three months. It did exactly what yours is doing. Finally I dumped PepBoys and tried a different brand at Autozone. (I can't remember the brand name off the top of my head but if I think of it I will post it) Anyway after that I never had the problem again.

kev

 

the brakes have to be bleed out with a star tester that ford has because you have rabs for abs braking and it has special bleeding process to be done
good luck

 

What is this star tester? How does it work? Can you purchase one through an auto store? I have rabs on my car, non-ford, and I was always able to bleed the brakes the same way as a non-abs car.

thx,
kev

 

I think I found my problem see below

False or Dropping Pedal
90ford24
WAGNER BRAKE CENTER

BRAKE FAX

TECHNICAL TIPS

Bulletin No. 24

Date November, 1989

Vehicle Make/Model

DODGE FORD and GENERAL MOTORS TRUCKS 87-90 BRONCO BRONCO II/RANGER F,E Equipped with Rear Anti-Lock Brake SystemFord: 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.