Upgrading BRAKES >_<

^^As above.^^ Adding a prop valve won't help you stop if you cannot lock up one end or both ends of the car. All the prop valve does is to "adjust" so to speak the pressure at the rear to somewhat less than the front

Might be a bad booster or a vacuum problem

Might be wrong master

Do you have a fairly hard pedal?

Might be drums are worn so to not contact the shoes properly (bell shaped, gouges) or they might be worn / turned too LARGE which makes the shoes contact only in the center portion of the lining.

Likewise the rotors may not be parallel, might need turned

Shoes / pads might be glazed

Other problems......swelled stuck shut hoses, junk and rust in system, stuck calipers. "Just generally." When was this work done, how old is it, and was the hydraulics flushed?

Consult a shop manual. You can download them free at MyMopar. Some of those there came from the guys HERE

You can get the disc brake info you need from the 73 shop manual

http://www.mymopar.com/index.php?pid=31