I hav a1991 ford f350 with a 7.3 in it.............i have replaced the brake booster, master cyliinder, front calipers and pads, rear shoes, wheel cylinders and drums. I have great pedla when the engine is not running, but as soon as i start the engine the pedal slowly goes to the floor all the way. I figured maybe the rabs valve or the vaccum pump but i replaced both and still does the same thing. I f anyone has some information or has had the same problem please tell me what you did to remedy it. Thanks,
Charles

 

If you have no external leaks, change the master cylinder again.

I have replaced the master cylinder on mine before and the replacement was worse than the one I took off.

When you get the new one, fill it with fluid before you install it. Then pump the fluid out through the fittings the lines hook up to. I have done this before and had a cup full of metal grit that was not removed after it was machined. The grit tears the seals up in very short order and the fluid bypasses internally and the pedal goes to the floor slowly.

Lack of quality control on the manufacture process is at fault.

 

have you checked the condition of the rubber lines, if the lines are swelling they are soon to pop

 

If you are pushing "very hard" on the pedal and it slowly creeps to the floor...... stop doing that it's not normal proceedure.

Pump the pedal quickly a couple times engine running, if the pedal comes up it could be an internal seal in the M/C as suggested. remember about 4 pumps and the brake vacuum is bled down on our diesels.

 

also, make sure to use a NEW master cylinder. the rebuilt ones are junk.

 

why when i put the trunk in reverse to adjust the rear brakes do i keep a nice hard pedal???? but when i put the truck back in drive the pedal goes to the floor?? will the position of the shoes in the rear make any difference to the perormance of the pedal??

 

Sounds like you may need to adjust the rear brakes if it makes that much difference.
The automatic adjuster may be stuck and not adjusting the brakes at all.

Yes if they are way out of adjustment it will cause the symptoms you listed.

 

I had a very similar problem on my 86 250. I had to replace the rear wheel cylinders and the spring kit. I also wound up replacing the vacuum pump. It took a few minutes of driving after I replaced the vacuum pump before things felt back to normal.

 

Don't throw your money away on the OEM brake setup... It will only fail again. Convert to hydro-boost brakes from a newer model Superduty. You will LOVE them. Best swap a guy could do in my opinion..... Thats my $0.02. I had the same problems with my 89 and could never get the brakes to be satisfactory until the swap.

 

The swap to hydra boost is a good one, but if you can not get salvage parts it is rather expensive.