Disc Brake System: Description and Operation

Disc Brake Caliper (Rear View):

Disc Brake Caliper (Front View):

COMPONENTS
This vehicle is equipped with single piston, slider type disc brake calipers. A 78.7 mm (3.1 inch) diameter caliper piston is used for all applications. The calipers are mounted in an adapter attached to the steering knuckle. The adapters support the caliper and maintain fore and aft alignment of the caliper and brake shoes. Non-metallic caliper pistons are used for all applications. The pistons are made of a phenolic resin material. The resin pistons are not interchangeable with steel pistons. Ventilated disc brake rotors are used for all applications. The rotors are serviceable and can be machined to restore surface finish when necessary. This vehicle is equipped with 325.6 mm (12.82 inch) rotors. The outboard brake shoe has flanges that locate and position the shoe in the caliper. The inboard shoe is positioned in the caliper adapter. The caliper is a one-piece casting. The piston bore is located in the inboard side. A square-cut piston seal is located in a machined groove in the cylinder bore. The caliper piston dust boot prevents dirt, water and road splash from entering the piston bore. The boot is seated in a groove machined at the outer end of the caliper piston. The boot retaining flange is seated in a counterbore machined in the outer end of the caliper piston bore.

OPERATION
The disc brake caliper is free to move laterally on the adapter slide surfaces. When the brakes are applied, fluid pressure is exerted equally against the caliper piston and all surfaces of the caliper piston bore. Pressure applied to the piston is transmitted directly to the inboard brake shoe pressing the shoe against the rotor. At the same time, pressure applied to the caliper bore surfaces makes the caliper slide inward. The inward movement presses the outboard shoe lining against the opposite side of the rotor to complete braking action. Any application or release of brake fluid pressure causes only a very slight movement of the caliper and piston. At brake release, the piston and caliper return to the non-applied position. The brake shoes do not retract an appreciable distance from the rotor. The minimal running clearance between the lining and rotor provides the advantages of improved response and reduced pedal travel. It also helps in preventing dirt and foreign material from lodging between the shoe and rotor surfaces.

Fig. 2 Piston Seal Function:

LINING WEAR COMPENSATION
Normal lining wear is compensated for by extension of the caliper piston and by lateral movement of the caliper in the adapter. Piston position is also determined in part by the square cut piston seal. Normal disc brake lining wear will cause the caliper piston to extend enough to maintain proper pedal height and brake response. The caliper bore will receive the extra fluid needed to compensate for the additional piston extension. As the piston extends during brake application, the square-cut seal is deflected outward. When brake pressure is released, the seal straightens and returns to a normal relaxed position. Although the amount of seal movement is quite small, it is enough to retract the piston to the necessary minimum running clearance. As lining wear occurs, fluid level in the front brake reservoir will decrease. This is normal and only requires adding enough fluid to restore proper level. However, when the brake shoes are replaced and the caliper pistons bottomed in the bores, the added fluid must be compensated for. This is necessary to avoid overfill and overflow. Removing a small amount of fluid from the front brake reservoir will prevent this condition.