Check that magnet is actually getting voltage.
Hold a compass next to that wheel down low where magnet is located
Then have someone pull the Emergency Breakaway Switch Pin
If magnet is being energized the compass needle will deflect towards magnet
(Pulling Pin allows FULL Battery Voltage to be applied and Checks that Emergency Breakaway Switch works-----don't leave it out longer than necessary to test)
Bet that wire connectors for that brake assembly magnet are loose/corroded or wires are chafted and magnet has not been getting voltage