brake adjustment

Its not easy. You have to get two tools in there, one to hold the locking tab up out of the way and the other to turn the adjustment star. IIRC you have to pry up pushing the star down but to be sure look in there with a
flaslight. The teeth on the stars on the self adjusters and tiny and asymmetrical, designed to engage with the locking tab, so that the tab will slide over the teeth when tightening and engage to keep the starts from backing off. So there's nothing to really pry against when trying to loosen them.

in other words, they are designed to be diabolically hard to back them down. So much so that I never bother, I just remove the drums, it's not worth the frustration and crawling under there.

Okay. The way the brakes work is that the electromagnet pulls against the rotating drum. The drag moves the shoes. When you are sitting still, the magnet will pull against the drum, but because there is no rotation, there is no movement of the shoes. As to why the one tightened up so much, I can't really explain it. I have had that happen to me also.

Check to be sure you have full voltage to both magnets. Turn the gain all the way up on your brake controller and have someone manually activate it while you check the voltages.

If you have a DC clamp on then you can check the current to each magnet also. Put the clamp on far away from the magnets though or the magnetic fields will mess up the clamp on measurements.

If the two magnet readings are close and the magnet sliding surfaces and the mating drum surfaces are smooth then it's got to be the brake mechanisms themselves that are causing the problem. They are after all pretty crude 1940s/50s technology.

You can clean up any places where things move and the threads in the adjusters and use a little (very little) brake lubricant, see if that helps.

Or just buy complete new brake assemblies ($62 each on Amazon) and hopefully be done with it....

+1. Much more modern system but complex and just too much money.

Barring that, I think I'd rather just have simple manual adjusters next time. Easier to set and to back off if needed. You just need to jack up the wheels every three thousand miles and set them, then check with the IR thermometer that they're not heating up when not activated and heating reasonably uniformly when activated. Should be good to go.