Brakes on F-350 4x4, Diesel

We have a 1996 Ford F-350 4x4 with a Powerstroke diesel that has been giving us a hard time with the brakes. Here is the problem, poor brake pedal- we can pump the brakes when the truck is off and the pedal is firm but when the truck starts the pedal fades to the floor. We have replace the following items- rear shoes, hardware, and wheel cylinders, master cylinder, vacaum booster, front pads and rotors (10k miles ago), check the for good vacaum, the rabs anti-lock dump valve. It think we have replaced or look at everything in the truck but the problem still prosists. We have no fluid leak either. We just don't know what to do. Can anyone think of any other thing that would cause this problem. Let me know. You can email me or call me at the shop @ 973-764-7600.

 

Make sure the rubber hoses arent swelling ,have someone look at them as you pump it up.Did you powerbleed the system or pump it up and crack em method?Sometimes all it needs is a powerbleed,could be a little trapped air somewhere-thats what it sounds like.Are your rear brakes adjusted tight?When your bleeding them by the pump it up method,try to notice which wheel has more fliud come out when the pedal goes to the floor after cracking the bleeder,when you find one that is less than the others-bledd that one extra long and see if you can get a little air out of it,Good luck