Pardon the musings of a tech who has seen some bad things happen in a relatively short career.
I saw someone with a brake failure just last week, where the wheel was so hot you couldn't touch it. At 3 feet, you would feel the radiant heat through your pants. The root cause was a stuck brake. The frightening thing was that this overheating situation was at city speeds (30-40mph) for about 4 miles. Had they been on the interstate, it would have been much worse. If the grease had cooked out of the bearings, they would have lost axle lubrication. This is how weels just "fall off" a camper at highway speeds. I know at least one person personally who had this very thing happen to them. She heard something, felt drag and she slowed down. Milliseconds later she saw a wheel wizz past her passenger side, bouncing off into a farmer's field. It virtually destroyed her camper.
Replacing individual parts in trailer brakes, like the magnet, is rare and usually only done in emergencies, such as when the magnet is mechanically damaged but still functional. Even then, it's often not cost-effective because the magnet's price is high compared to replacing the whole brake assembly, and the way the spindle works to take of the entire brake assembly at that point is 4 more bolts. Its just not worth the headache to try part fixes.
Trailer brakes are more complex than a car's hydraulic brakes. They rely on mechanical components like levers and friction parts, beyond the brake shoes.
Here's how it works: when you activate the brakes, an electric current turns the magnet on (like wrapping wire around a nail to make it magnetic, as you might recall from school). The magnet sticks to the inside of the spinning brake drum, pulling a lever it's attached to. This lever pushes the brake surface against the drum to slow it down.
Over time, the magnet and drum wear into each other, forming a unique pattern, so they become a matched set. If you replace one, you usually need to replace the other to ensure proper function, as they rarely stay perfectly flat. Other parts, like springs and components, wear out with age and use. Because of this, replacing the entire brake assembly rather than individual pieces is usually better, as everything works together as one unit.
The ol' pad-slap brake job that you might do on your ol' jalopy can create a serious dangerous situation for a trailer. Probably not so much for a 1500 pound Camp-Inn. But most DEFINATELY is an issue with a 8,000 or 10,000-pound monster being pulled behind a half-ton pickup. Heck, I've seen 'full size' towables behind SUVs. But I assure you, the salesman said it was perfectly safe.
