rear brakes dragging sometimes

Hello and thanks for any help/ideas, here is my problem, the rear brakes (drum) drag to the point that the drums get so hot they burn the paint off, but like this morning I drove approx. 20 miles no problem, then on the way home they heated up big time, this is what I have done, all new/reman master cylinder, both rear wheel cylinders, both front calipers, newer SS brake lines, new prop valve, the lines and prop valve were replaced 5 years ago I got those from in-line tube, so right now the only things I haven't replaced is the power booster, and the prop valve> (it's 5 years old though). at this point I have not made any progress other than I have almost all new parts!!