HYDRAULIC TO MECHANICAL FORCE
Disc brake calipers squeeze the pads against the rotors when the brakes are applied. Calipers come in two basic types: Floating and fixed. Floating calipers are the most common and are so named because they slide sideways on their mounts to center themselves over the rotor when the brakes are applied and released. Most have a single piston behind the inner brake pad, but some have two or more pistons.
Floating calipers ride on slides, bushings or pins. This hardware must be corrosion-free, undamaged and lightly lubricated so the caliper can move, otherwise it will stick and cause uneven pad wear (usually the inner pad wears more). Fixed calipers do not move and have pistons on both sides of the pads (usually one or two on each side). Uneven wear in a fixed caliper can be caused by a sticking piston.
Caliper service is required if a caliper is leaking brake fluid, if a piston is sticking or frozen, or if the pads show uneven wear.
Many brake experts recommend rebuilding or replacing the calipers on high-mileage vehicles when the brakes are relined for preventive maintenance. Like any other mechanical component, calipers wear and corrode with age. Every time the brakes are applied, the back-and-forth motion of the caliper pistons produces a slight amount of wear on the piston seal and piston bore.
PREVENTING CONTAMINATION
Moisture contamination in the brake fluid adds to the problem by allowing corrosion to attack the piston bores and the caliper pistons if they are steel or aluminum (which is why the fluid also needs to be changed every few years!). As the surface of the pistons and bores become pitted and rough, the seals wear even more and may begin to leak.
Also, as the pads wear, the caliper pistons move gradually outward. This exposes more of the piston to potential contamination and corrosion from dust or moisture that may get past the dust boot. If an old corroded piston is then shoved back into the caliper when new pads are installed, the piston seal will be riding on a rough surface. It won’t be long before the seal fails and the caliper starts to leak.
Brake fluid leaks are serious because fluid can contaminate the brake linings, causing them to grab or pull. Fluid loss can also lead to brake failure.
Even if a caliper isn’t leaking, it is still aging. Rubber piston seals and dust boots lose elasticity with age. Square-cut piston seals help retract the pistons and pads when the brakes are released. If the seals are old and hard, they may not pull the pistons back, allowing the pads to drag against the rotor. When dust boots get old, they often crack or split and allow dirt and water to enter the piston bore area. The result can be accelerated seal wear, piston corrosion and sticking.
Rebuilding or replacing the calipers when the brakes are relined restores the system to like-new condition, improves brake reliability and reduces the risk of leaks or other problems that might lead to a comeback and an unhappy customer.
