Hi guys. I am new to the world of FORD's I recently picked up a 1986 F250 Dually with the 6.9 idi and 4spd, I have brake light on in dash and spongy pedal but it seems that my front brakes are on ( jacked up and can't spin the wheel by hand) ..... I am a heavy duty mechanic and usually work on air brake systems so the hydraulics are a little confusing right now lol

 

Hi, it sounds like something wrong with the master cylinder or proportioning valve. The brakes light can trip usually if the hg vacuum pressure is low or the proportioning valve is out of position.

That you have spongy brakes and not stiff points to the master and not the booster. Though do carefully check for broken vacuum hoses and repair them, also look over the cruise control vac hoses.

But most likely it's the master cylinder, the proportioning valve usually resets itself when this is working properly. How that works is it regulates the front/rear hydraulic pressure differential, ie front 65 back 35 percent and at varying conditions.

Because if it were 50/50, you can lock up the brakes and would spin out of control.

When one reservoir has a problem or is empty past master cylinder piston, the pressure differential has a fail safe, so it doesn't lose all brakes front and back. It slides an internal valve shut for the circuit not working. And sends a brake light. This will usually reset automatically when the brake circuit is repaired and you will have all 4 brakes instead of 2. It usually takes a few minutes or days to right itself.

When my truck was new to me had a wheel cylinder leak down and drain the master resovoir to the rear. Causing the proportioning valve to shift and brake light on. What I did was disconnect the sensor wire from the valve to turn off the dash light, verifying that was it and the rear circuit was off.

I fixed the wheel cylinder and refilled the resovoir and reconnected the valve. Dash light stayed on a while, days I think - but I don't remember and then it decided to reset and all works.

Please note, many master cylinders from the store are garbage. Some folks get 2 bad ones in a row and think that there is another problem. Don't go cheap on the master cylinder, and as far as brake brands in general, I always go with Centric.

Good luck, stop back by and let us know how it's going okay. Oh hey also, use DOT 123, never 4 or 5, brake fluid. Newer is usually not improved and brake fluids are no exception. 4-5 will give a soft spongy pedal and be sure to bleed them very well.

Running on, but bring an IR thermometer and flash the rotors and hubs to make sure that you don't in fact drag your brakes, because the fluid will boil to a vapor, first fade, and then any pedal response (see Fred Flintstone using feet to stop).

 

The Rear ABS valves (RABS valve) can also bypass internally. You will have the same symptom as a bad master cylinder, spongy pedal etc.

 

Orlen, you'd be wise to take Perk85F250's good advice. The ABS valves are a known problem, years ago Kelsey Hayes, Bendix or whoever manufactured them came out with a service bulletin stating they were prone to failure. The pedal will slowly sink under constant pressure like a bad master cylinder.

 

RaymondIV and Perk85F250 thanks for the help but as far as I have seen under my 1986 F250 there is no abs

 

Leroy Unlisted thank you for the where to start ..... so far I have found; rear left wheel cyl. bleed screw rounded off, front right caliper bleed screw broken off, front right caliper seems to be seized up, unplugged proportioning valve connector and light went out ...... I'm not sure if I just have air in lines that previous owner could not bleed out properly or if master is gone .... am going to start by changing out caliper and wheel cyl. and check out all brakes .. possible full brake job if needed, then see if master is any good....... wish me luck lol