This exact same problem just came up recently in another thread, and the problem was the RABS module. These modules can fail in a number of ways - one of which is that it can leak off brake line pressure when it isn't supposed to (such as when you are sitting at a stop with your foot on the brake) which will cause the brake pedal to slowly drop to the floor.

I'd bypass the RABS module with a section of steel brake line with the appropriate fittings and see if the problem goes away. Then you know for sure, and many people drive around w/o rear ABS anyways.

 

ok, an update. i bypassed the RABS module with brake line, bled the system properly, and now still has the same issue. i have a firm pedal with the key off, i have a firm pedal with the key on to the ignition setting, but as soon as i start up the engine, the pedal sinks. my father and i pulled the booster vaccum hose with the engine running, the pedal returns. I am thinking that I have a faulty booster, i just wanted to verify that there wasn't anything else i needed to investigate first.

 

one last thought, could the check valve, located on the end of the vaccum hose, be the culprit? could i have been chasing my tail for months just to solve it with a $6 part??

 

The check valve just holds vacuum in the booster.
If your pedal sinks and stays down with the engine running you have a defective booster, or a mis-adjusted rod.

 

another booster, same issue....

I think im going to jack up the rear and check rear drum adjustment JUST TO BE SURE ....

 

Did you figure out your problem? I am having the exact same issue with my '88 F350--bout ready to drive it off a cliff!

I replaced the booster with a used one (same thing) new master (same thing), new calipers, rotors, verified lines are clear, bled till I was sweating brake fluid-(same thing)....still have a soft pedal with engine running--pumps up great with engine off---soooooo frustrating!!

please let us know!

thanks!

Kevin

 

I'm still fighting it ... got sidetracked on another project

 

I know the feeling... too many projects, too little time...but I need this truck operable by Wednesday.... darn brakes on this truck have been driving me to insanity for 3+ yrs now, almost ready to take it in to dealer and let them pull their hair out! But I hate admitting defeat, especially on something as seemingly simple as brakes!

I'm ready to attack this problem again now, I'll let you know if I am successful...