Surge braking systems usually make it safer and convenient to travel with a trailer. This is because they not only provide the trailer with a way to brake, which makes braking the tow vehicle easier and safer, but also eliminates the need of a driver having to juggle two braking systems: his car's and the trailer's.
Surge brakes usually work by relying on the difference between the speed of the trailer and that of the pulling vehicle. This difference usually occurs during braking, in that applying the pulling vehicle's brakes usually slows it down. However, it doesn't slow the trailer. This usually creates a "push" effect at the hitch area.
This push effect is what usually activates the braking. It does so because the surge system's "brake pedal" is located at the hitch assembly area. And since the push effect caused by the differences in speeds is almost similar to the one caused when you press against the brake pedal, it activates the trailer braking system.
The surge braking system problem
However, a problem usually arises when one is reversing. This is because reversing creates a push-like effect. This effect then creates pressure in the braking system, something that activates the brakes. This then makes it almost impossible to reverse since the more you try to reverse, the harder the brake shoe will press against the drum – that is, the more the braking effect.
There are different ways that manufacturers take care of this problem. The use of an electric solenoid valve is a common solution.
Using an electric solenoid valve
The surge braking system usually relies on brake fluid to transmit pressure to the hydraulic cylinder - which eventually causes the braking effect. If something interferes with the flow of the braking fluid, it will definitely affect the functioning of the braking system.
Manufacturers usually manipulate the brake system's reliance on braking fluid to change how this pressure is transmitted. They do so by using an electric solenoid valve. They connect this valve to the reverse lights in such a way that every time one reverses, the valve messes with pressure transmission. It therefore helps to keep the trailer system's brake from engaging when it is in reverse mode.
However, if the wiring between the reverse lights and the solenoid valve is tampered with, the inbuilt prevention system will fail. It will also fail if the valve is faulty. Therefore, if your trailer brakes engage when you are reversing, valve-related problems may be to blame. Simply having the valve changed, or having the wiring issues sorted out, should be enough to get your surge braking system working as efficiently as it should.