1974 Duster with manual disc brakes

Hi,
I have a '74 Duster that was ordered with the manual front disc brakes. It is a very low mileage car and I'm getting it ready for good weather driving this up coming Spring. The problem is that the rear drums lock up the rear wheels with anything more than slight pedal pressure. Theres no leaks in the system (master cyl is full) and the brakes work, just with an unwanted screech from the rear tires....Any help appreciated! thanks