American Motors: AMC brakes, rear brakes, drum brakes

Question
I have a '69 Javelin SST, 343 CID, auto with drum brakes. I cannot keep rear brakes on it. I can bleed them, and clean the valve body (proportioning valve?) and they will work for only a couple of weeks at the most. Is the problem the valve? I didn't think this did anything except on a car with disc brakes on the front and drum on the rear, but I don't know what else it could be. I have replaced booster & master cylinder before. there are no leaks.  

Answer
What do you mean when you say cannot keep rear brakes on it?
Do the rear brakes not apply? Do they wear out? If they are not applying, how did you test them? Do you get a strong flow of fluid when you bleed the rear cylinders? If they don't apply, have you inspected the inside of the wheel cylinders for corossion? Please let me know. By the way, if you have 4 wheel drum brakes, your car does not have a proportioning valve, it probably has a pressure differential warning valve which turns on a light with a pressure loss in a line.
Kevin