If properly used, the compass can tell you that the magnet is working.
Hold a compass close to hub, below center but above bottom of drum. The magnetic needle will point to hub (No Zee, your fone will not work) Move compass away until needle points back north. Have helper apply brakes, the needle should swing back towards drum. Now this will not tell you everything, but will tell you the circuit is good enough for magnet to move the needle.
Now the OP has not reported smoke, bearing or tire failure, so it is safe bet when let off brakes, all are released.
You and 3 of your friends are carrying a pool table. 1 stops to take a phone call, can you and 2 handle the load? Maybe, but lot more strain. The OP has 1 brake on the phone.
Electricity likes to be where it's at. If it can't see a way to get back, it won't go anywhere. A open circuit means there is no way for it to go around. There is no open on the 3 that work
Decades of working on automotive electrical, most problems come from using the chassis for ground, so the first thing I would look at would be where the wire hooks to ground. It might be a short, but most likely somewhere between where that wheel circuit branches off and ground there is a open circuit.