Clueless on brake problem, pedal to floor
I am completely clueless on what to do next. Problems started when the right front brake pads wore out on my 1996 F-250 4x4 because the caliper was sticking. I turned rotors, resealed calipers and replaced pads. After putting everything back together and bleeding air out of system I could not get a good pedal with the engine running. Pedal was firm with engine off and after starting the engine brake pedal would slowly creep to the floor, so figured I must have damaged master cylinder while bleeding. Replaced master cylinder with rebuilt unit and decided to do rear brakes at the same time with new shoes, hardware and new wheel cylinders. Put everything together, bled system and still couldn't get a good pedal with engine running. Pedal is hard with engine off, however slowly goes to the floor with the engine running, acting just like an internally leaking master cylinder. I am bleeding using a vaccum pump (also tried manually) have bled all starting at RR, LR, rear ABS valve, then go to RF and LF. Master cylinder was bench bled before installing and complete system bled several times and still had the same problem. So, I thought something must be wrong with the rebuilt master cylinder and installed a brand new one, ended up with same, hard pedal with engine off, slow creep to floor with engine running. I read on the forum that a bad brake booster could cause this, so put one on today, no help.
Vacuum pump pulls 19.5" hg in 10 seconds with engine running. There are no external leaks, more than a gallon of brake fluid has been put through the system, everything except the vacuum pump and the rear ABS valve have been replaced. I can plug the ports on the master cylinder and get a normal pedal with engine running. If I reconnect the rear brakes everything is still good, connecting either or both fronts and the problem comes back.
Sorry for the long post, but I'm at a loss on what to do next. Anyone have this before or know the fix?