I have a 73 road runner that can't get a pedal on the brakes. I have replaced the lines, wheel cylinders, master cylinder, calipers, pads and shoes. When you have the car running and pump the brakes a pedal will build up after 5 or 6 pumps. I have bleed the brakes untill no air is is in the lines, but there still isn't a pedal on the first pump. When driving down the road it will go all the way to the floor and lock up the back brakes. I think I have the power booster rod adjusted correct. Can a prop. valve go bad?

Check the calipers and make sure the bleeder valves are located at the highest position.

also the adjustment on the rear brakes can cause a problem, but not usually 5 pumps.