No direct experience, but a hydraulic surge brake system is just like the brakes
in a car, but with no power assist, and with a linkage in the coupler instead of a brake pedal. The master cylinder in the coupler works exactly like one in a car, and would be rebuilt the same way. For many cars, it is cheaper to replace the master cylinder than to rebuild it; I have only replaced them.
In addition to boat trailers, just about any rental trailer heavy enough to legally require brakes
is likely to have the same type of surge brake system, so many trailer fabrication, rental, and repair businesses will be quite familiar with them.
If you can pull the master cylinder out and take it with you, I think a good auto parts person would be valuable - they may be able to match it, despite its trailer origins.