disc brake problem.....

I have a 71 Duster that came with factory disc brakes. I rebuilt the front calipers, replaced the hoses, and put a new master cylinder on it. My problem is that I cannot get any fluid to the back brakes by pumping the pedal. I disconnected the linee at the rear distribution block and pumped the brakes and got nothing. I then took an air gun and blew the line out and sprayed the brake fluid out of the resevoir. Still didn't get any fluid out. Is there something I am missing here or do you think i have a bad prop valve? This is all that is stopping me from putting the car on the road, no pun intended.:dontknow: