I am working on a 92 F250 Super cab with a 7.3IDI engine. I use this primarily for plowing snow. I have driven this truck for roughly 5 years, and I have owned it for the last 2 years. The rear brakes are drums and the front are rotors. The brakes have always been a little iffy and last year they got worse. Almost all the brake lines are only about 2 years old other than the brake hoses to the calipers which are about 4 years old. They are still flexible and are not cracked or anything like that. The master cylinder was replaced last summer. Over the last week, on the rear brakes I put in new wheel brake cylinders, new shoes, all new hardware, and new drums. On the front, I put on new rotors, new calipers and new pads. I have completely bleed the brake systems using a brake bleeder (is uses a compressor to create a vacuum). When the engine isn't running I get plenty of brake pedal and they feel really good. When I start the engine, the pedal goes away, and it fades to the floor. If I put it up it comes back but then fades to floor again. If I leave it sit running for a while, the first time I hit the pedal it has no real resistance and goes to the floor until I pump it back up. The vacuum line to the booster doesn't appear to have any holes and I can't hear any normal whistling from it like you can with other vacuum leaks. This is pretty much what was happening last winter by the end of the winter. Anyone have any idea of what else I can look at? Thanks in advance.

 

First thing i would check would be the rear abs unit mounted under the drivers firewall on the frame. These like to fail in their old age. There is a bleeder valve on it. I ended up bypassing mine. I had similar symptoms with the addition of sticking rear brakes occasionally.

 

I will check that, but I am 99% sure that was removed out of the truck as the ABS light stays on, and I have traced this lines from the master cylinder all the way to the back and don't remember seeing anything but straight brake lines. I know the proportioning valve was removed but I don't believe that should cause this problem.

 

RABS valve is a big one, check that and also make sure you adjusted the rear shoes properly. If you have new or turned drums then you should barely be able to slide the drums on or off when properly adjusted. If they have a bit of a lip you'll have to hit the star wheel from inside the brake housing at the access hole.

For the RABS valve that light can come on whether it's installed or not, BTW. Let us know how it goes

 

The RABS valve is no longer on the truck. It has been by-passed. I confirmed by looking at my other F250 that still has it and it is still on the truck. To my knowledge the rear brakes are also adjusted correctly. I have already put the tires back on but they didn't spin easy nor freely after I let go. They pretty much stopped after I let go of the wheels.

Someone and I can't remember who said that diesels have a vacuum generator on them that could cause brake fade if it isn't functioning correctly. Is there any truth to this? I am not super familiar with diesels as this is my first one so I am still learning on the fly.

 

The vacuum pump and assist make the pedal easier when working properly-- if something in the vacuum system isn't working right the pedal will feel harder, not sink to the floor.

If the rest of the brake system is working right then it sounds like your master cyl is bypassing and needs to be replaced.