Hydraulic braking systems heavily rely on the effectiveness of pressure transmission from the brake pedal. The vehicle's brake pedal is like lever on a pump which when pushed causes the pump to create pressure. This pressure is then transferred to the wheel cylinder, which then forces the vehicle's brake shoe to press against the brake drum, causing the braking effect.
The surge brakes on your trailer work the same way. The only difference is that it does not rely on brake pedal pressure. Instead, its braking mechanism solely depends on the momentum and weight of the trailer and essentially works by mimicking your vehicle's speed.
When traveling at normal speeds, both your trailer and your vehicle move at the same speed. If you hit the brakes, your vehicle's speed will reduce. The trailer will however be traveling at the same speed as before. This mismatch in speed will trigger the trailer's pump mechanism, which is conveniently located as part of the hitch assembly, that then causes the trailer to brake. The more the mismatch in speeds, the more the pressure that is generated and the harder the braking.
Once there isn't a mismatch in traveling speeds, the "pump" system should be able to stop generating pressure and thus causing a release of brake fluid pressure, something that should release the brakes. Sometimes, this doesn't happen. This is so especially after a making a sudden stop.
Here are some of the things that may be causing this to happen.
A clogged restrictor orifice
A restrictor orifice is a device that regulates the flow of brake fluid with the sole purpose of preventing sudden on-off-on-off braking that can lead to pulsating movements. It simply slows down the rate at which the brake fluid travels and thus making for a smooth braking process.
If the restrictor orifice is clogged, it can act as a valve. The clogging can hamper the flow of the braking fluid and hence keeping the brakes engaged when it's not supposed to.
To solve this problem, simply clean the restrictor orifice. And if that doesn't work, simply replace it with a new one.
A mechanical problem
The trailer's brakes can also get stuck after sudden braking if there is a problem with the surge actuator slide assembly. If it has jammed, which may be due to rusting or normal wear and tear, it can prevent the master cylinder piston from moving back to its relaxed position, something will definitely cause the brakes to get stuck.
The same can also happen if the master cylinder's piston gets stuck. And also if the system's push rod, which affects the movement of the master cylinder's piston, is bent or it is improperly adjusted. Simply replacing worn-out parts and adjusting any parts that are out of place should be enough of a brake repair to solve any of these problems.