How does a Hydraulic Brake work?

The hydraulic brakes on your trailer are much like those on your car. The hydraulic fluid from a master cylinder is used to actuate the wheel cylinder which, in turn, applies force against the brake shoes and drum. The main differences between automotive hydraulic brakes and hydraulic trailer brakes are the actuation systems which transfer the braking signal from the tow vehicle to the brakes.

The surge braking system uses a specially designed trailer hitch coupler which has a hydraulic cylinder built in. When the tow vehicle applies its brakes, the tow vehicle decelerates causing the trailer to apply a pushing force against the hitch. This force actuates the surge hitch hydraulic cylinder, transferring high pressure brake fluid to the wheel cylinder. The trailer brakes are now applied.

This type of hydraulic brake utilizes a single-acting cylinder. Upon actuation, the primary shoe is pressed against the brake drum, which causes the shoe to move in the direction of rotation. This movement in turn actuates the secondary shoe through the adjuster link assembly.

Another variation is called a « free backing » brake which is commonly used on trailers with a surge hitch system. When backing with a surge brake hitch, the brakes are applied through the surge mechanism, and if there is more brake force on the trailer than the tow vehicle can override, no backing is possible. The free backing brake was developed to allow backing in this application. This brake has a primary shoe on a pivot which allows normal application in the forward direction, but allows the primary shoe to rotate away from the drum surface when backing.

Uni-servo	Duo-servo

The duo-servo brake uses a dual piston wheel cylinder to apply the brakes. This type of brake is typically used in a vacuum/hydraulic, electric/hydraulic or air/hydraulic system. A description of operation of this brake is as follows.

When the brakes are applied, the double-acting wheel cylinder moves the primary and secondary shoes towards the drum. The frictional force between the brake drum and lining attempts to turn the primary shoe into the secondary shoe. The secondary shoe is forced onto the anchor pin and from this point, the secondary and primary shoes attempt to « wrap around ». In essence, the brake has utilized frictional force to help the applying force on both shoes.

If the brakes are applied while the vehicle is backing, the shoes rotate in the direction of the drum rotation. This causes the secondary shoe to leave the anchor and causes the primary shoe to move against the anchor. Action of the brake is therefore the same in reverse as forward.