Brake problems: I have a 1993 Ford F350XLT Diesel 7.3 Intl, 1 ton dully crew cab. I have not been able to get a full brake pedal. Have bled the brakes correctly, replaced master cylinder three times, booster, and equalizer. The master cylinder when plugged, you get a full solid pedal, with NO bypass, but when lines are hooked up, you get a bypass, and the brakes slowly sink to the floor. I have had more than three shops look at this, and they are all dumb founded.

Any help please!!!!