my 86 had no rear brakes as the rear brake cylinder let go. I replaced everything and cannot get any fluid to the rear brakes when I bleed them, I changed the combination valve with another used one and still no fluid. Could the master cylinder be bad? I am not sure how long the car has had no rear brakes I bought it that way.

 

could the lines be rotten somewhere between the master cylinder and the back brakes? try and follow the line and see if fluid is leaking somewhere along the way

 

Nope, already tried that. All new line to the back. The only thing that has not been replaced yet is the rubber hose from the rear line to the tee, and the tee itself. Fluid will flow thru from gravity, but no pressure when pumping the brake pedal.

 

A bad proportioning valve possibly? I had to change the one on my 87 ss because i had too much pressure to the rear brakes, but i imagine they can go both ways.

 

It could be some O ring into master cylinder worn, but first chek lines for bending or stuck, open bleeder screw, you now that.