System Description (General Description)

Applying the brakes causes the wheel cylinder piston to force the leading edge of the primary brake shoe and lining assembly against the rotating brake drum. The primary brake shoe tries to rotate with the drum and transfers force to the secondary shoe and lining assembly through the adjusting screw assembly. The secondary shoe's lining leading edge grips the drum and tries to rotate just like the primary shoe. Since the shoes cannot rotate, they wedge themselves into the drum. The rotating torque from the shoes increases the braking force applied by the wheel cylinder. Because of this wedging action, the design is a duo-servo (as opposed to a single-servo design where the wheel cylinder pressure alone is the source of braking force).

The brake shoes transfer the torque through the backing plate to the axle flange. Brake adjustments are automatic and occur during reverse brake applications.