Three basic designs of hydraulic brakes are used for service brakes for trailers: non- servo, duo-servo, and uni-servo.
The non-servo brake has two shoes pivotally anchored at their bottom ends, and a double ended cylinder arranged to spread the shoes apart at their top ends. When hydraulic pressure is applied to the cylinder the shoes are pressed against the drum. The friction force resulting from rotation of the drum tends to increase the pressure of one shoe but to decrease the pressure of the other. While this provides a braking action that is quite smooth with little tendency to grab, the non-servo brake has a relatively low efficiency and is therefore not recommended.
In the duo-servo 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 the 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. It will readily be seen that the duo-servo brake will work equally well in either direction. While the duo-service brake is inherently less stable than the non-servo brake, smooth operation is achieved by the proper selection of brake lining materials. Probably the best argument in favor of servo type brakes is the fact that they are used almost exclusively on passenger automobiles.
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, but it is not required with the AEROL surge system that permits the equipment to be operated in the reverse direction.
AEROL CO., uses the duo-servo type brake exclusively for service brake installations. Hydraulic line systems are also available to connect the brakes to the hydraulic surge unit on the tow-bar. A parking brake feature is provided by a cable linkage to the secondary shoe and a lever actuator that permits remote control.