Brake Part

A brake pedal that feels like a rock is pretty much normal for drum brake Mopars. The drum brake cars didn't use a proportioning valve. They had a combination valve that was basically a T for connection purposes. The only real function it performed was to sense low pressure in one circuit and trip the brake warning light on the dash. How old is your rear brake hose? Sounds like it is breaking down inside and acting like a reed valve. Pressure can go into the brakes but none can return to the master cylinder. It's either the hose or the master cylinder, the combination valve won't cause the problem you are having.