In operation, when the tow vehicle slows, the inertia of the trailer presses the actuator against the coupler, pushing the piston into the master cylinder. Like stepping on the brake pedal in a car, this applies pressure through the brake lines to the individual brakes at the trailer wheels. The system self-regulates the amount of braking force because the force on the piston is proportional to the rate of slowing of the tow vehicle.
The more the tow vehicle tries to slow, the more force is applied to the piston and through the hydraulic fluid to the trailer brakes. Conversely, when the tow vehicle accelerates, the hitch ball pulls the coupler forward, extending the piston and releasing the brakes. A very simple, cool design.
However, an unintended consequence of surge brake systems is that the actuator also applies the brakes when backing up because the tow vehicle is pushing against the inertia of the trailer. On flat ground with a light trailer, this may not be a problem. However, it can be a big problem when backing up a hill or driveway with a heavy trailer, especially if the trailer is equipped with disc brakes instead of drum brakes. Therefore, the brake system requires a method to deactivate the brakes when in reverse.
Some actuators allow the driver to deactivate the brakes manually by temporarily inserting a lockout pin into the actuator to keep the actuator/coupler mechanism from compressing. However, the pin will prevent brake actuation both in reverse and in the forward direction. This presents a serious safety hazard because if the driver inadvertently leaves the lockout pin in place while driving forward, the trailer brakes will remain deactivated.
A better approach is to use a system that automatically deactivates the brakes only when backing up. In this respect, drum brakes have an advantage over disc brakes because the design of the brake shoe mechanism in drum brakes leverages much of its braking force from the forward rotation of the drum. As a result, drum brakes are very ineffective in reverse, so backing up is not difficult even if the brakes are applied. In addition, some drum brakes utilize a "free-backing" design, which virtually eliminates all braking action in reverse.
Disc brakes, however, are equally effective in forward and reverse because the braking force is independent of the direction of rotation of the rotor. To prevent braking in reverse, actuators intended for disc brakes often utilize either a blocking or bypass solenoid. The solenoid mounts between the master cylinder and the brake line, and connects to the backup lights of the tow vehicle. Therefore, the solenoid actuates only when the vehicle is in reverse gear, eliminating the potential of the brakes remaining deactivated when driving forward.
As the name suggests, a blocking solenoid blocks the brake line, preventing pressure from the master cylinder from pressurizing the brake line. However, a blocking solenoid can still result in the trailer brakes activating while in reverse because the solenoid will not relieve existing pressure that can already be in the brake line. This will occur if the driver stops while traveling downhill and shifts into reverse to back up the hill. Even though the blocking solenoid will prevent the master cylinder from applying additional pressure to the brake lines, the existing residual pressure in the brake lines can keep the brakes applied.
As an alternative, a properly designed bypass solenoid relieves the pressure from both the master cylinder and the brake lines by opening a bypass port into the brake fluid reservoir. This prevents the master cylinder from applying new pressure to the brake lines, and relieves any existing residual pressure in the brake lines.