The uni-servo brake is similar to the duo-servo design, but a single ended cylinder is used, and only the secondary shoe rests on the anchor. As a result, only slight braking action is developed in the reverse direction. This feature is desirable when used with some hydraulic brake actuators that make no provision for backing the trailer.

In the duo-service brake, the two shoes are connected at their bottom ends by a floating link and impinge on an anchor at their top ends. A double acting cylinder is located adjacent to the anchor so that when hydraulic pressure is applied, one shoe will be actuated by the piston and the other will bear against th anchor, depending on the direction of rotation of the drum. The floating link transmits the piston force and the friction force developed by the primary shoe to the secondary shoe providing a highly efficient braking action.