I agree with the previous posts. Return springs fatigue through the life of the original shoes. They provide resistance that if lessened, allows the pressure from the proportioning valve to press the shoes against the drums harder since the springs offer less resistance. Good return and hold down springs help this.
In a rarer situation, a proportioning valve that is letting too much pressure to the rear wheels can create a lockup or grabbing condition. This however would require a set of Brake pressure gauges to diagnose and most don't have a set hanging around.
I would agree that proper assembly and adjustment should be checked out.
New adjusters that are properly lubed will also help. Sometimes GM uses a small V shaped clip in the adjuster. It is a bi-metal spring that expands when hot to prevent over adjustment of the rear shoes. If yours had this, it would be a good idea to replace it along with the springs.
