Hi Pete, the voltage varies as to how hard the brakes are being applied, anywhere from 0 to 12VDC, If the magnets are working and the brakes are not activating then maybe your brake controller isn't working. Good luck GB
With meter probes one on the hot wire at the trailer brake and the other on the ground wire (do not use the frame as a ground) at the trailer brake. With your engine at idle and a fully charged battery, apply full manual brake with the lever on your controller. You should have no less than 10.5 volts at the trailer brakes. Poor connections are the biggest cause of low voltage.