Ford F-350

I have a 1989 Ford F-350 7.3L Diesel 5spd front disc, rear drum (duals) that has around 130k miles on it. The problem I'm having is when I apply the brakes they stop the truck fine, but with continued pressure on the pedal it will very slowly travel to the floor. I was told the problem was the ABS unit (mounted to the inside of the frame, left side of the truck) so I replaced it (with a rebuild) but that didn't fix it. I replaced the master cylinder (a new unit from ford) and that didn't do it either. A friend of mine removed the hose from the booster and blocked it with his hand while I depressed the pedal and that seemed to hold, so I replaced booster but that didn't help either. I've been over the truck front to back and there are no leaks and the fluid level hasn't dropped in the resevoir throughout this adventure. The front pads and discs have a few thousand miles on them and the back shoes and drums are new. Assuming the parts I installed were good, I'm at a loss as to what the problem is, so any suggestions are greatly appreciated.
June 6, 2007.

Pull up on the parking brake. Does this solve the sinking pedal? If yes, adjust the rear brakes, if not try replacing the check valve on the booster! Also what happens if you keep pushing the pedal? Does the light come on? Lose all brakes? What?

Jun 7, 2007.
Thanks for your reply. By " pulling up the brake" I assume you mean setting the parking brake while pushing on the brake pedal. If I got that right, there's no difference in braking and I can't feel any difference in the brake pedal as I set the parking brake, the pedal still goes to the floor. No lights come on and the brakes will continue to hold the truck on a mild grade, so the brakes don't fail when the pedal gets to the floor. Of course, I've only tried that with the truck empty, loaded might be a different story. Typing this I can't see your reply, but I think you mentioned replacing a check valve in the booster? The booster that I installed was more or less a unit and I'm not sure where the valve is located. Could you elaborate on that for me? Also, it occurrs to me that in installing the ABS modulator (not sure if that's the correct name) and the master cylinder, both were bled, but is it possible that air might be in the lines beyond either of those two parts? The pedal builds slightly when pumped a few times, not much, but I wondered if there might be air in the lines that I'm compressing when I push on the pedal causing it to travel to the floor. Thanks again for your help.

Jun 7, 2007.