Disc surge…. I’m used to surges with drums! I’m also used to electric over hydraulic braking systems. Those typically have disc brakes. Please post pictures of tow hitch, electric connection to truck, and wheel hubs both sides with wheel removed.

did you buy this trailer new or used? If used, wonder if seller install wrong components for your trailer.

Basically, the hydraulic system is retaining not releasing the pressure.

Have you jacked the axles off the ground? Can you turn the wheels? Chances are no. As the pads are pressing on the discs al the time.

its pointing to clogged system.

1. The orifice is clogged on the output side of the orifice. The clog is acting as a “check valve” allowing the fluid to come out of the fitting, but won’t let it go back in.
2. The surge actuator slide assembly has mechanically jammed (stuck) in the compressed position & will not allow the master cylinder piston to return to it’s relaxed position-thus preventing the fluid from returning to the reservoir & releasing the brakes.
3. The piston in the master cylinder is stuck in the compressed position preventing the internal return spring from pushing the piston to its “parked” position-allowing brake fluid to return to the reservoir & releasing the brakes.
4. The steel push rod that pushes the master cylinder piston is adjusted too long or is bent & will not allow the master cylinder piston to return to it’s relaxed position-thus allowing the fluid to return to the reservoir and releasing the brakes.

its going to take some work to fix this. I’d start by draining the system, then seeing if the brakes have released.

 

Likely you are having to ride the brakes in a hilly area. Continually activating the brakes. The activator needs to be loosened or made less sensitive, or the brake shoes loosened.
On my first cheap brake controller, non-proportional, the brakes kept locking up and overheating riding brakes on downhills. Replaced it with a proportional type, deglazed the brake shoes and all was well. Granted, these were electric but the symptoms end up the same.

 

If your trailer has disc brakes and somebody put on a surge brake actuator made for drum brakes, that could be your issue. Drum brakes require a residual pressure in the lines after the actuator is released. If you use one made for drum brakes with a disc system, it will cause the disc pads to always have a little pressure against the rotors.