Disc Brake System: Description and Operation

DISC BRAKE COMPONENTS

Disc Brake Caliper:

All models are equipped with dual piston, slider type disc brake calipers. Dual 54 mm (2.126 inches) diameter caliper pistons are used on 1500 series model. Dual 56 mm (2.2048 inches) diameter caliper pistons are used on 2500/3500 series models.

The caliper is a one-piece casting. The piston bores are located in the inboard side. A square-cut piston seal is located in a machined groove in the cylinder bore.

The caliper pistons 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.

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.

The 1500 series models are equipped with 298.4 mm (11.75 inches) rotors. The 3500/2500 series models are equipped with 325.6 mm (12.82 inches) rotors.

DISC BRAKE OPERATION
The disc brake caliper is free to move laterally on the adapter slide pins. 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 pad pressing the pad 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 pad 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 pads 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 pad and rotor surfaces.

DISC BRAKE 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.

Fig. 2 Piston Seal Function:

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 pads 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.