For several months I have felt that the rear brakes weren't working probably which seemed to be cornfirmed by the fact that the front pads lasted only 18 K miles. I have bled the rear system several times but never seemed to get the rear braking feel while driving. Since the truck had 220K miles I felt that the master cylinder was probably suspect and bought a reman unit from NAPA. While I had the old one off I was hoping to see an obvious seal tear or split in the secondary piston but saw nothing in the seals that would indicate any problems. After I put the new one on (bench bled of course) I still had problems with the rear brake feel and thought that maybe I introduced air in the ABS unit when I broke the lines but I did "burp" the fittings when I reconnected them. Reading some of the posts one method (w/o the special tool) of bleeding the ABS unit was to lock up the wheels on dirt or gravel to purge the air from the ABS unit. I did this and then started the normal bleeding procedure RR, LR (engine off) but all of a sudden according to my daughter who was pumping the brake pedal the pedal went "real soft". Now I have almost zero pedal with engine running and boost assistance and engine off just a slight pedal. It feels like there are no seals in the M/C. I can't even pump up the system like you can with air in it to build up pressure. Other than something radically going wrong with the M/C is there something in the ABS unit that could cause this condition ?