rear brakes dragging sometimes

[412 Stroker]

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!!

 

[draginmopars]

what about the rubber line> from rear steel brake line (frame) to rear axle splitter

 

[412 Stroker]

5 years old SS from inline tube, we cracked the line lose from where the ss line and the hose goes together and the brakes released making us assume it was the master cylinder so I replaced that but it is still doing it. so it's not the hose.