![]() ![]() Consequently, when new pads are installed, the caliper piston must be fully seated in the caliper housing to allow the new pads to slip over the disc rotor. When the bleeder screw is broken off in the caliper housing, the caliper assembly usually must be replaced.īrake caliper pistons are designed to compensate for brake pad wear by extending as the brake pad lining wears away. Road salt may also cause the caliper bleeder screw to seize in the caliper housing, which prevents the technician from bleeding the caliper during a normal service procedure. Road salt also causes the floating caliper to seize on its guide pins and cause uneven wear rates between the inner and outer pads. Rust corrosion eventually causes the caliper piston to stick in an applied position against the brake pad, which results in excessive and uneven brake pad wear. Continued exposure allows moisture to contaminate the brake fluid and further corrode the piston and caliper bore. As the boot fails, dirt migrates under the piston seal which, in turn, creates a wear groove on the piston surface. Although the caliper piston is sturdy enough, a torn rubber boot often exposes the piston to dirt and corrosion. The caliper piston boot is the brake caliper part that fails most often. ![]() In the real world, however, brake calipers live in a very hostile environment that includes heat damage from extended braking, mechanical damage caused by road debris and, of course, corrosion damage caused by wintertime humidity and road salt. Theoretically, disc brake calipers should enjoy a very long, trouble-free service life. All in all, modern disc brakes contribute very much to the quest for high-performance, fuel-efficient vehicles. Engineers also began using sealed wheel bearings with zero endplay to control and reduce pad-to-rotor clearance. To improve fuel economy, engineers designed caliper piston seals that retract the caliper piston a few thousandths of an inch from the brake rotor. To reduce pad-to-rotor clearance, disc rotors are machined to less than a few thousandths of an inch runout and less than a few ten-thousandths of an inch variation in rotor thickness. The clearances between the brake pad and rotor are therefore very critical to establishing correct brake pedal height and response. A brake pedal travel of one inch, for example, may be required to move the front caliper pistons just a few thousandths of an inch. Because a tremendous amount of pressure must be applied to the disc brake pad, the ratio between brake pedal and caliper piston travel is very big. Unlike drum brakes, disc brakes generate no servo effect and therefore must rely solely upon hydraulic pressure to force the brake pads against the rotor. Because modern vehicles are heavier and use smaller-diameter wheels than lightweight racing vehicles, caliper design has evolved from expensive multi-piston designs to the single-piston, hydraulic or vacuum-assisted floating caliper designs in use today. To illustrate, early racing and high-end vehicles used large-diameter rotor, multi-piston, fixed caliper designs that had enough swept braking area to stop the vehicle without using a conventional vacuum assist.
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