Question about 1994 Ford F250 Supercab

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I have a 1994 F250 4X4 with a turbo diesel engine that has several brake issues. Several years ago I had the brakes serviced. The brakes worked normally at first with a couple of exceptions. The first is that if you press on the brake pedal, while stoppped, a little harder than required, the brake pedal will slowly go to the floor and the brake light will go on. The is no air in the system, I have had that check several times. The second is that the brake light comes on at different, apparently random times. Sometimes it comes on and goes off as if you were turning it on with a switch. Other times it fades in and out as if you were using a rheostat. Lately it has been acting as if there was no power assist. *hard pedal, lots of pressure with significantly reduced brake effectiveness. As iff you turned off engine and tried to stop) intermittantly. The frequency of this is increasing, and guaranteed if you pump the brake pedal once. Where should I start looking to troubleshoot this?

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  • Scott Graham Sep 08, 2010

    I suspect that this is a compound problem that includes the vacuum canister, contamination, internal master cylinder malfunction or some combination.

    Vacuum appears to be ok, but no gauge to measure, just reasonable(?) at vacuum canister and manifold. No change in rpm when I disconnected vacuum line from brake vacuum canister or when I plugged line with thumb. That would indicate a vacuum pump as opposed to using intake vacuum. Vacuum felt same at manifold (a separate distribution manifold, not the intake manifold) and brake canister.

    As much as I dislike the shotgun approach, and unless you can provide additional info to chase this down, when you consider disassembly time and what goes into addressing each individual problem, I think the way to go is:
    Replace brake vacuum canister
    Replace master cylinder
    Flush brake lines and bleed brakes

    Any thoughts?

    E Brakes inspected every year via disassembly for state safety inspections with no discrepancies noted. They were rebuilt 7 years ago with all cables replaced. The parking brake appears to be functioning normally No ABS light issues have been noted.



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The problem you are describing is the exact symptoms of a faulty master cylinder. This may occur while holding your foot on the brake pedal at a stop light. If the pedal goes slowly down, it means the master cylinder is not holding pressure. This is also a potentially dangerous condition because a worn master cylinder or a leak in the hydraulic system may cause the brakes to fail. It will also cause your brake light to flutter on and off

Posted on Sep 09, 2010

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Check your Vacuum Reserve Cannister for rust. This part is like a Coffee can with vacuum fittings. Then check your Vacuum pump for performance. You should have some pressure at idle, I do not know your models specs but I would hope its in the Teens.

With the engine running use a water spray mist from a spray bottle around your Vacuum Brake Booster. Actually, try spraying every vacuum line and if there is a leak the engine will change RPM as it draws in the water. There should be no leaks.

While there may not be air in the Brake system, there may be condensation or a contaminant. You may also have a Master Cylinder leaking fluid within. When you push down on the Master C, the fluid pressure is suppose to stay downward of the valving. If the valving leaks, it allows the pressure from the High pressure side to return to the reservoir without you lifting your foot off the pedal.

The gradual brightening of the warning light can be a dirty contact inside the warning sensor. Or it can be a corroded plug connector on the outside of the sensor. The ABS warning light can be showing you pad problems and fragmented, loose pads can do this.

Finally, the Emergency brake pads, whether used or not, can rust off and roll inside the drum portion of your rear Rotors. The Brake shops often will not check this unless they are pulling off the rear Rotors. The E brakes are hidden and can not be seen without disassembly.

Would like to know what you found out about this problem.

Posted on Sep 08, 2010

  • fordexpert Sep 09, 2010

    Still would check Brake Booster to save money, but its there with the Master Cylinder off, so it would be quicker to do both at once.

    Try checking the the Vacuum Reservoir by putting a vacuum line to it and inhale. They censor the other word. Its a small chamber anyway and you can find whether it leaks using lung power.

    As I understand it, Diesels need the Engine Vacuum pump because they do not generate manifold Vacuum. A different Brake Booster won't hurt. My State does not do Annual Inpections unless they run the DOT signs on the cab. Good that you know the E brakes are inspected.

    Glad that you are looking at several of the repairs I mentioned.



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