I have had basically the same problem on a single wheel. First test run and the small amount of air in the hub superheated and blew off the dust cap. Also almost pushed the inner grease seal off. But part of that problem was that the drum was slightly oversized compared to the other three,
Anyway, even with the wires cut to the single magnet, it still overheated. Problem seemed to be with the magnet arm, not enough clearance between it and the cross bar between the two shoes AND the magnet was tilted such that the leading edge of the magnet would grab on the stud holes and therefore slightly apply the brakes even w/o power. I bent the arm and the tang that the magnet rides on. Of course you can't see what is really happening inside the drum, so it took several tries to fix the problem.
Texas Baptist Men-Retiree Builders member since '01
13 Silverado 3500HD D/A, 2wd CCSB srw, custom RKI bed
11 Cruiser CF32MK