You can 'test' if magnets are energizing by holding a compass next to wheel and then activate brakes by puling emergency brake cable out of switch (Applies full battery voltage)....compass needle will deflect when magnet energizes
With a meter you can test voltage to each individual brake assembly at wire connections when activated.
You can 'test' braking by jacking each wheel up, spinning it and then activate brakes
Might just be time for adjustment of brakes.
I can only 'drag' my 5vr on dirt/gravel.
NOTE: DON'T PULL EMERGENCY CABLE OUT WITH TRUCK/TRAILER CONNECTED
Some brake controllers can be damaged