|JIM’S CORNER – AUTO REPAIR HELP|
THE BRAKE SYSTEM
There are two basic types of drum brake lining arrangements used on vehicles. Servo and dual servo brake systems. Dual servo brakes are a self energizing type of drum brake system, mostly used on older domestic vehicles. The brake shoe arrangement uses a primary shoe and a secondary shoe connected at the bottom, using the automatic brake adjuster screw. The primary shoe uses a smaller amount of lining than the secondary shoe and the material is usually softer. When the brakes are applied, the rotation of the brake drum causes the top of the primary shoe to be pulled forward in the same direction as the brake drum. This causes the primary shoe to apply pressure to the secondary shoe, through the brake adjuster screw. This action is called self energizing since it creates its own force to assist the brake hydraulic system in applying the brakes. Single servo brakes simply use the wheel cylinder alone to force the brake shoes against the drum surface. Both the leading and trailing shoe are usually the same size and manufactured out of the same material.
The brake shoes are mounted on a plate that is bolted to the rear axle housing, on rear wheel drive vehicles or to the rear spindles, on front wheel drive vehicles. The mounting plate is referred to as the backing plate. Older model vehicles with four wheel drum brakes will use a backing plate attached to the front spindles, as well. The brake shoes are attached to the backing plate using spring loaded retaining pins. The spring loaded retaining pins are used to allow lateral movement of the brake shoes for brake application. Refer to an appropriate auto repair manual to see diagrams for your particular vehicle.
The brake shoes are held in a retracted position by the brake return springs. There are many different configurations of brake return spring arrangements, depending upon the systems design. The brake return springs can be anchored at a central point at the top of the backing plate and attached to the brake shoes. Some vehicle drum brake systems use a single spring to connect the leading brake shoe to the trailing brake shoe. Some systems use a large rigid metal bar as a brake return spring. The bar is bent into a U shape, with the upper ends of the U attaching to the top of the leading and trailing shoe. The brake linings are retained in a closed position by the tension of the metal bar. Contained in the drum brake system hardware, are provisions for automatic adjustment of the clearance between the brake linings and the brake drum. Automatic brake adjuster designs vary widely rom vehicle to vehicle. Most older domestic vehicles use an automatic brake adjustment system that operates when the brakes are applied while in reverse. Most recent designs use an automatic brake adjuster system that operates when the parking brake is applied.
(Jim is a lifelong fan of Dodger Baseball and used to race sprint cars in the 1980s.)