Derek,
Some background please:
How many brakes?
On how many Axles?
Get a compass.
Track the brake wiring and inspect it all.
There are often connections near the backing plate that can be separated. If you have those, you are in luck. If the brake control is two separate wires anyplace, put the compass near each and have someone step on the brake. by comparing the result, you will know which is the shorted magnet.
If it is a shorted magnet, you will probably have to pull the drum to replace it (or repair it if you can identify the issue).
Matt
