PLEASE HELP!!!

[dajain]

I have a '95 PS F-250 and what I am having problems with is the brakes.

What is happening is when the motor is not running, the system stays primed and pedal stays firm. When the motor is running, it's a different story. It has the stopping power at first, then the pedal slowly sinks to the floor and it looses brakes. This is what I have done for the last 2 days:

Of course, the first instinct was the master cyclinder. Bought a rebuilt from the parts store, bench bled it, and installed. Bled the system and still doing the same thing.

Next, I isolated between the front and rear system. I plugged the front port and left the rear attached. Problem went away. Pressure stayed with motor running. Reversed the situation, plugged the rear and attached the front. Problem came back.

I then replaced the left caliper, problem still present.
I then replaced the right caliper, problem still present.

I then started to test vacuum:
-Brake booster held 15" for 20 minutes with no leakage.
-put vacuum on entire system and system held for a longer period that what it takes for the brakes to bleed off
-Vacume pump read 24" while running with no load. Allpied the brake and the pressure dropped under 5" and took a little longer to recover

Due to the drop and the "whooping" noise the pump was making, I decided to change out the pump and check valve. Did not corrct the problem

I then connected the vacuum from the pump directly to the booster thinking about bleed off from other functions. problem still there

I them decided to return the master cyclinder and replace it again thinking I got a bad rebuilt. No such luck

Talked with several mechanics about it and they kept saying to bleed the system. I've done that 4 times already with no change either way

I then discovered that this system has the ABS system on it. I did a standard bleed on the ABS valve on the frame no change. It makes sense that this would be my problem. Hydraulic works fine without motor running, then acts up when motor (& computer) is running. The vacuum system is working perfectly now. Only thing left is the ABS.

All this for one question?!?! How do I bypass the ABS to see if this is what is causing my problem? Or, just bypass the valve on the frame? Can I just get rid of that valve and put a manifold block in its place?

I've already spent way too much trying to fix this problem (even ask the wife) and just want to get it fixed cheaply now

I will be hauling a 10K pound 5th wheel ASAP and need the stopping power. Any suggestions would be great.

Thanks, Dave

[okney1lz]

This must have a leak somewhere. One that only shows up when there is the added assistance of the booster.

Maybe in the line that runs the length of the truck?

Pull the wheels and check you calipers and wheel cylinders for leakage.

[dajain]

What has been dont to the truck to this date is as follows:
Replaced the master cylinder (Twice)
replaced the front calipers
replaced the rear pistons
replaced the front brake lines (Steel)
replaced the vacuum pump and check valve
removed the pressure spring from the RABS valve
removed the fuse for the ABS
Not to mention all the times that the system has been bled

There is no physicial evidence of any leak in the entire system. I sat there and pushed the pedal for about 10 minutes and there was no puddle or wet spot There was one little wet spot onthe front line and that is why I replaced it.

All this and the pedal still sinks to the floor and it looses brakes

My next move is going to be to just to completely remove the RABS valve from the system

Woulds really love some clues as to what is wrong with this truck!

Thanks for the reply,
Dave

[melmount]

subscribed, heres a bump

[okney1lz]

The RABS could be dumping into its accumulator any time it sees pressure, which would give the symptoms you describe.

So I agree, take the RABS out and see what you have. Or replace, but personally I think the Rear ABS is useless.

Don't forget to bleed in this order: RR, LR, RF, LF. Maybe gravity bleed them first then do a manual bleed?

Let us know.

[melmount]

where is the rabs located? I had the same issues with a excursion
do you just plug something to eliminate this or remove it entirely?

[okney1lz]

It's located on the inside of the drivers side frame rail just forward of the fuel selector switch.

Just follow the brake line down and ya can't miss it!

I would yank it completely, but that's just me. It seems as if he has already disconnected it and still has a problem. So it is the only logical thing that could be causing his problem.

[dajain]

Originally Posted by okney1lz

It's located on the inside of the drivers side frame rail just forward of the fuel selector switch.

Just follow the brake line down and ya can't miss it!

I would yank it completely, but that's just me. It seems as if he has already disconnected it and still has a problem. So it is the only logical thing that could be causing his problem.

I am going to stop at the local hardare store tonight, get a 3/16" brass compression coupler, and just take the thing out of the system.
I really don't care if the ABS light stays on, it will eventually burn out. As long as the truck has brakes again. I'll be very happy

[dajain]

OK, I bypassed the ABS valve for the rear tires. Started the truck and the problem is still there. The pedal still sinks to the floor.
The only other device in the system was the booster. Even though it tested good with a hand pump, I still replaced it.
Gravity bled the system for 3 hrs on each rear wheel, and 2 on each front.
Started the truck, and problem is still there! WTF!
2 master cyclinders, both front calipers, front brake lines, both rear cyclinders, bypassing the ABS valve, a new booster, and no wet spots to be seen.
Unless someone has an idea, I am just going to have to live with it.

Thanks for all the replies so far.

[okney1lz]

The only other things I can think of:

Rear brakes severely out of adjustment, but I doubt it as many times as you've had this apart.

The flexible line between the frame and axle in the rear is bad and forms a bubble when pressure is applied. Crazy, I know. But it would explain the fade and no loss of fluid.

Maybe get a flare cap and isolate the front and rear one at a time to try and find where the problem is.

This irks me, something has to be wrong, the fluid has to be going somewhere when you press the pedal.

Ciao,

Lawrence