Help!! '87 S-10 brake problem

[Opie2211]

I have an '87 S-10 with manual brakes. When I bought the truck it had been sitting for almost 2 years and the rear brake line (along the frame) was busted, so it only had front brakes. Well, I replaced the line and when I tried to bleed the brakes, I got no fluid at all. I tried taking the line directly off the master cylinder to see if it would come out there but it didn't. So I bought a new master cyl. and it did the same thing...no rear fluid. I thought it was bad too, so I took it back and got an exchange and still yet, after another new master cyl., no rear brake fluid pumping through the system at all. My only guess left is the proportioning valve. Could this be the problem? Maybe stuck in full front assist mode? Any help would be greatly appreciated. Thanks!!