Drum Brakes (Rear)

DESCRIPTION
The parking bra All models are equipped with rear drum brake assemblies. They are two-shoe, duo-servo units with an automatic adjuster mechanism.
Two different size drum brake assemblies are used:
- 1500 models: 11 x 2.25 inch
- 2500/3500 models: 12.125 x 3.5 inch
All vehicles use a 1 inch bore wheel cylinder.
The drum brakes are a semi-floating, self-energizing, servo action design. The brake shoes are not fixed on the support plate. This type of brake allows the shoes to pivot and move vertically to a certain extent.

OPERATION
In operation, fluid apply pressure causes the wheel cylinder pistons to move outward. This movement is transferred directly to the brake shoes by the cylinder connecting links. The resulting brake shoe expansion brings the lining material into contact with the rotating brake drum.
Two forces affect the brake shoes once they contact the drum. The first force being hydraulic pressure exerted through the wheel cylinder pistons. And the second force is the friction generated turning torque of the rotating drum.
The drum forces both brake shoes to move in the same direction of rotation. Servo action begins with the primary brake shoe which begins to wedge (or wrap) itself against the rotating drum surface. This force is transmitted equally to the secondary brake shoe through the adjuster screw and anchor pin. The net result is that each shoe helps the other exert extra force against the drum. It is servo action that creates the wedging (or wrap) effect which produces increased force on the drum braking surface.
All drum brake assemblies are equipped with a self adjusting mechanism. The components forming the mechanism consist of the: adjuster screw, adjuster lever, actuating lever (11 inch brake), lever return spring and the adjuster lever spring. The adjuster lever on the 12 inch brake, is also equipped with a lever and tension spring.
The adjuster mechanism performs two important functions. First, is in maintaining proper brake shoe operating clearance. And second, is to maintain brake pedal height. The mechanism does so, by adjusting the shoes in small increments to compensate for lining wear. The adjustment process is continuous throughout the useful life of the brake lining.
The adjuster components are all connected to the secondary brake shoes. Actual adjustment only occurs during reverse brake stops. Secondary brake shoe movement (during reverse stops), is what activates the adjuster components.
In operation, secondary shoe movement causes the adjuster lever spring to exert pull on the lever. This pivots the lever away from the adjuster screw teeth. When the stop is completed and the brakes released, the adjuster lever pivots back to a normal position. It is during this return movement of the lever when adjustment occurs. At this point, the lever comes back into contact with the adjuster screw teeth as it moves upward. The lever will then rotate the adjuster screw one or two teeth as needed for adjustment.

NOTE: The adjustment process requires a complete stop to actually occur. Rolling stops will NOT activate the adjuster components. In addition, the adjuster screws are left and right hand parts and must NOT be interchanged.