GM-GMC: truck brakes, differential switch, truck brakes

Question
Randy      On a 96 chev c2500 6.5 turbo diesel, the brake line broke along the chassis while plowing. The master cylinder was emptied, front chamber. I wound up putting on a new master cylinder because I figured I had to remove it to prime it again anyway. Primed the new cyl., repaired the line. Still only get 1/2 pedal. I can't get pressure to backs to bleed. There is weak pressure at master cylinder front line,(rear wheels?) Strong pressure at rear line on master cylinder.  I hope I explained all that well enough .                  Thanks for any input.        Jim

Answer
Jim, when the line broke on your truck the pedal surely went all the way to the floor. when this happens the presure differential switch would have been set. This stops the flow of fluid to the front or rear brakes to help maintain some brakes for the remaining good half.  the rear chamber on the master is for the frt brakes.  To reset the switch  open driver side frt caliper bleed screw and hit the brake petal really hard to force the switch back to center and then re-bleed the complete system starting at the right rear, then left rear, then right frt and then left frt. You may have to repeat this several times to ensure the pedal becomes firm.