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Keith Wayne
Keith Wayne, ASE Certified Technician
Category: Ford
Satisfied Customers: 131
Experience:  16 years in Field. Ford Engine Master, Chassis Master, and Diesel Specialist
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1990 Ford F350: 7.3 diesel..brakes..new booster..brand new

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I have a 1990 Ford F350 7.3 diesel 5 speed dually.   I'm having trouble with the brakes. It's had new wheel cylinders, new pads and shoes, new booster, and I've tried three different Ford brand new master cylinders. The vacuum pump seems to have plenty of vacuum. After I drive the truck for about a day, I lose pedal and the ability to stop. If I pump it, it will stop but not well. If I bleed just the back brakes, I can regain pedal for another day. No one in my area seems to know what's wrong. Can you please help me.

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

 

 

 

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 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.

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Customer: replied 7 years ago.
Keith,
I forgot to mention I replaced the brake proportioning valve with a second hand one from a salvage yard. It made no difference and I am not losing a single drop of fluid. Does that change anything? Customer
Joe This is not the porpotionong valve. This valve is on the Anti lock brake system. There is an acculator piston that absorbes the pressure. As designed, it only opens when commanded by the ABS system to unlock the rear wheels. But when the valve gets stuck open the pressure from the pedal is forced to fill up this space, when you let off the pedal there is a spring behind the piston that forces out the fluid. So next time you apply the brakes it has to fill it up again. You can find it by tracing the rear brake line, it has the line in and out. three wires i think. and a bleed screw on the top.