pull the drums off and get clean everything down with break clean. Brake dust and other things build up in the drums and cause this. At least this is what had caused it when I ran into it a few years back on a few other GM cars.
correct.Pull the drums off and take a look at them.They are probably 10 dollar shoes. takes some 200 grit sand paper sand the shoes a little.Clean everything with brake clean. Make sure you do not have any seepage from your wheel cylinders. You should be good to go.