The number one killer of trailer brakes is rust. Generally, the surge brake housing is galvanized or zinc plated. If the shocks inside leak, or the master cylinder leak at all, the fluid will eat the zinc coating and rust begins. One thing with the shock is that the exposed rod will rust, then when it is compressed, it tears the seal and the leaks begin.
Another source of leaks is the back up cut off solenoid. This is usually screwed into the output of the master cylinder and is not protected very well. It is prone to being knocked into shifted and torn loose causing more leaks.
If your hydraulic lines are rubber, well, storing outside in the weather will trash these lines pretty quickly. This is a double edged sword, the solid steel lines are prone to rust and break fairly easily due to the flexing of the trailer, the rubber lines rot....but in my experience, the rubber lines are much easier and cheaper to replace.
Disc brakes are arguably the best. Biggest problem is they will rust in a minute, unless you buy the super expensive stainless ones or the coated ones. Same with the calipers. Even so, with disuse even these quality parts will lock up.
This also happens with drum brakes.
Lack of use and sitting out in the weather will lock up the calipers, rotors, wheel cylinders and shoes. Every time the surge brakes put pressure on the locked up mechanism, the fluid has no place to go and will seek the weakest part to work on, break, and leak.
Guys will spend hundreds of dollars on a boat cover, and totally neglect their trailer. Best advice is to prevent damage that causes the leaks.
Cover the operating parts when not in use, and every month or so connect the trailer and drive it around a little.
