One Brake Drum Very Hot

A slight dragging won't make the brakes that hot. It is much more likely that the cold drum is a indication that the brakes on that side are not working properly, even though the magnets appear to be activating both sides with the wheels off the pavement. Brakes are supposed to get hot when you use them, that's how they dissipate the energy from the moving vehicle to slow it down.

I had the cold drum problem and found it was caused by imbalanced magnets by measuring current to the individual magnets separately. They were quite different, even with new magnets.

You can make this measurements with a clamp on dc current meter but you have to keep the the clamp far from the magnets or the magnetic fields will cause the clamp on Hall effect meter to be inaccurate. Best to put the clamp on up near the 7 way connector and then connect one magnet at a time.

That also gives you the chance to get rid of the crummy insulation displacement connectors foods have. Replace those either with good self sealing heat shrink butt splices or with soldered connections.

Once I determined that imbalanced magnets were the problem I disconnected the good brake and made several stops with the weak brake only, following Lipperts brake burnishing procedure, because the weak brake never got a chance to get burnished in properly while both were operating.

Then, to force the magnets to have the same current I rewired them in series rather than parallel as they are normally and turned the gain all the way up on my brake controller to compensate as much as possible for the lower voltage at each magnet from the series connection.

Even then my brakes were not fully balanced (as measured using an IR thermometer after a couple hard stops), but they were at much better.

Bottom line is that electromagnetic trailer brakes are crude, antiquated junk by today's standards and there is only so much you can do to get them to be balanced and work as they should. That's not to say you aren't right to try, I found that brakes that grabbed on one side more than the other were much worse especially on wet pavement, than ones that were balance but a bit weak.

To test the brakes, I connected the trailer to the truck, jacked up each wheel and had someone apply the brake controller while the wheel was spinning. The brakes stopped the wheel and then was allowed to spin after releasing the controller. The brakes worked on both sides. I am thinking that the cool side may be adjusted looser than the right?

It aounds like ypur brakes were sticking on the hot side or the cool side are not working at all.