My buddy called me up today saying his brake pedal was at the floor, and the red brake warning lamp is lit. He stopped by and I drove it. Felt as if there was a leak somewhere, so I checked the system out. No leaks anywhere. I even pulled the master away from the booster to check in there. I put the truck on the lift and had him drive it in 4WD and hit the brakes. The fronts worked fine, but I was still able to turn the rears by hand. They had a little drag when he was applying the brakes but not much. I figured the master must have gone bad so I installed a new, not reman, unit in it. Bench bled the master, installed it, then bled the whole truck from the farthest wheel to the closest. The fluid that was in the truck looked like dirty diesel oil, so I continued bleeding until everything came out clean. When bleeding I noticed that I would have to wait about a minute between bleeds at the rear wheels or I wouldn't get any fluid out. The fronts get fluid right away every time the bleeder is opened. Needless to say the pedal is still at the floor. The truck has RWAL so I bypassed the RWAL unit thinking maybe the dump or isolation solenoid was stuck, but there was no difference. I think it might be the proprtioning valve, but in 7 years as a tech I have only replaced one, and it was an electronic one on a Ford. I also though maybe the rear brake hose may be collapsing internally, but that usually keeps the rear brakes applied after releasing the pedal, or doesn't allow them to get fluid at all. Sorry this is so long winded, but I had to walk away from the truck after a few hours of getting nowhere. Any help is appreciated, so thanks in advance.