System Description Leading/Trailing Drum Brakes

Leading/trailing drum brakes are used on vehicles up to and including a 6450 pound GVW rating (excluding the Police Package). Both brake shoes are held against the wheel cylinder pistons by the upper return spring and against the fixed anchor plate by the lower return spring. When the brakes are applied, the wheel cylinder pistons move both shoes out to contact the drum. With forward wheel rotation, the forward brake shoe wraps into the drum and becomes self-energized.

With reverse wheel rotation, the rear brake shoe is self-energized. Force from the brake shoes is transferred to the anchor plate, through the backing plate, and finally to the axle flange. Adjustment is automatic and occurs on any brake application. It is normal for the front (leading) shoe and lining assembly to wear faster than the rear (trailing) shoe and lining assembly.

System Description Duo-Servo Drum Brakes

Duo-servo drum brakes are used on vehicles with a 6600 pound and higher GVW rating (including the Police Package). When the brakes are applied, the wheel cylinder piston forces the leading edge of the primary shoe and lining assembly to contact the rotating drum. The shoe tries to rotate with the drum and transfers force to the secondary shoe and lining assembly through the star-wheel adjuster. The secondary shoe's lining leading edge bites into 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 called a duo-servo, as opposed to a single-servo design where the wheel cylinder pressure alone is the source of braking force.

The torque from the brake shoes is transferred through the backing plate to the axle flange. Brake adjustments are automatic and occur during reverse brake applications except, on the Police Package model vehicles which adjust in the forward brake application.