184 Auto Brakes Copyright by Goodheart-Willcox Co., Inc. rotor. Th e loss of pad pressure allows the pads and caliper to “fl oat” back to the normal resting position. Th is action is shown in Figure 12-16. Fixed Calipers Th e fi xed caliper is rigidly bolted to the spindle and does not move during braking. Basic fi xed caliper operation is diff erent since hydraulic pistons are installed on both sides of the rotor. Fixed Caliper Construction Fixed calipers are usually made of cast iron. Some fi xed cali- pers are a one-piece design, Figure 12-17, while others are split so they can be disassembled for easier piston removal and honing. Fixed calipers are not commonly used except on heavy equipment and military vehicles. Some makers of high-performance brakes off er two- and four-piston cali- pers as replacements for original equipment calipers. Fixed Caliper Operation When the brakes are applied, hydraulic pressure enters the caliper assembly, Figure 12-18. Pressure fl ows through internal passages or external transfer tubes, pressurizing all four pistons at the same time. Th e pistons move outward, forcing the brake pads into contact with the spinning rotor, slowing or completely stopping it. When the brakes are released, hydraulic pressure decreases and the pistons are pulled clear of the rotor by the action of the piston seals. Rear Disc Brake Caliper Rear disc brake calipers do the same job and have roughly the same components as front brake calipers. However, they Bolts Spindle Splash shield Brake hose Adapter Inboard lining Guide pin Positioner Bushing Bleeder screw Caliper Anti-rattle spring Bushing Piston seal Piston Dust boot Outboard lining Hub and rotor assembly Chrysler Figure 12-15. Exploded view of a one-piston, floating brake caliper that bolts to the adapter. Housing retainer pin (locating pin) does not move with caliper Shoe and pad assemblies Piston Piston seal Caliper housing Bleeder Caliper fluid inlet Sleeve Inner bushing Steering knuckle Outer bushing Dust boot Applied direction of travel Released direction of travel Cadillac Figure 12-16. This shows the operation of a single-piston, floating caliper. As the piston is forced outward, hydraulic pressure (which is equal in all directions) forces the caliper to float (slide) inward. This movement applies the outboard pad.