Brake issue

Sounds like that's what is happening, but you might want to do the 'skid-pad' test I described just to make sure.

An easy way to check if the magnets are in working order it to check the continuity with an ohm meter.  Both of the magnets should have about he same resistance. 

Yes you don't need to disassemble.

If you get a multimeter with a Hall effect DC current clamp on you can measure the current to each brake. But don't place the clamp on near the magnets or the reading will be inaccurate.

If you don't have a clamp on then disconnect the crummy insulation displacement connectors to each magnet and measure the voltage there. Then replace with proper butt splices.

You can put your meter on the ohms scale and measure the resistance of each magnet while you have them disconnected. The Lippert brake manual gives you the resistance range.

I found that even though I had voltage to both magnets and both were in the resistance range I was getting lower current through the pass side magnet, enough lower that it never grabbed hard enough to burnish properly. I rewired my brakes in series so they had equal current and turned the gain on the controller all on the way up. Then an agressive burnishing. Problem solved.

Mine were imbalanced like that with new brake assemblies that I installed myself. No grease, just variations between magnets and the fact that the right side brake gets less current than the left one due to the longer wire run. And several folks have reported bad connectors resulting in one brake not operating. It's well worth doing the electrical checks first.