Trying to figure out my brakes

Hi all,
I have a question about a 72 Dart. My car came with power disc brakes according to the paperwork on the car that came with it new. My problem is the car locks up the rear brakes upon of any kind of stop that requries more pedal than a light push.
The pedal is also very hard and seems to sit up higher than normal.
My question is did these car come with a brake booster? The car doesn't have one on it and I would think it would.
I picked up the Dart from a person who was not really mechanically apt. Anyway while talking about the car he said that they tried to put the new style brake system on the car at one time but it didn't work out very well so they put the brakes back the way they came. I am thinking I may have some mismatched parts and would like to see what the car should have came with.
Thanks,
Kowality