180 Auto Brakes Copyright by Goodheart-Willcox Co., Inc. Pad-to-Rotor Clearance Modern disc brakes have almost no clearance between the lining and rotor when the brakes are released. Because residual pressure check valves are not used with disc brakes, no static pressure is present to apply the pads. Th erefore, even with very low clearances, there is almost no brake drag or wear. Low-drag calipers have greater clearance. Th is clear- ance can be as much as 0.006″ (0.15 mm) when the brakes are released. Th is is designed to remove all brake drag, which aids in fuel economy. Calipers using this design are always used with quick take-up master cylinders, discussed in Chapter 5. Brake pad-to-rotor clearance adjustment is automatic. As the pad linings wear, the piston moves farther past the seal. Th e seals can retract the piston enough to eliminate brake drag, but no more. Th erefore, as the pads wear, the piston moves out farther. Extra brake fl uid enters the piston chamber to compensate for piston travel. Th is is illustrated in Figure 12-8. Caliper Types Th ere are two methods of mounting disc brake calipers, depending on the hydraulic piston arrangement. Calipers are grouped as fl oating and fi xed calipers. On modern vehi- cles, fl oating calipers are the most common. Fixed calipers are used on a few modern high-performance vehicles. Th e two types of calipers are discussed in the next sections. Floating Calipers Floating calipers use one or two pistons, located on the same side of the caliper. Th e caliper can move or “fl oat” back and forth in relation to the rotor. Th is fl oating action is made possible by attaching the caliper in a way that allows it to slide on the mounting hardware. Floating Caliper Construction Floating calipers are commonly made of cast iron or alumi- num. Th e most common method of allowing the caliper to slide is by the use of mounting bolts that allow the caliper to move over them. Th is is shown in Figure 12-9. Th ese bolts Brake pad Outer caliper housing Brake shoe Piston dust boot Wheel mounting stud hub bolt Hub Bearing Steering knuckle Teflon seal Slinger Splash shield mounting bolt Caliper mounting bolt Inner caliper housing Piston spring Bleeder screw Seal Fluid passage Rotor Piston seal Piston Chrysler Figure 12-7. A brake caliper that is attached to the spindle (steering knuckle) with mounting bolts.