This might sound like a stupid question, but the magnet is supposed to be in constant contact with the face of the drum, right? This way, when it becomes energized / magnetized, it grabs the face of the drum, causing the lever on the magnet to swing, which causes the shoes to expand.
Another test that I had done was to have the truck hooked up to the trailer and with the trailer wheels off the ground, I checked the wheels to see if any of the shoes were hanging up or being energized with the foot off the brake. Each of the 4 wheels spun freely and locked up (trying to spin them by hand) when my Dad stepped on the brake.
