Any caliper that is leaking, has worn or damaged seals, or is causing brake pads to wear unevenly, needs to be rebuilt or replaced, but so do many calipers that appear to be trouble-free.
After three or four years of service, most caliper bores and steel pistons have visible corrosion and pitting. As the surface of the piston becomes rough, it starts to wear the piston seal. Every time the brakes are applied, the roughness on the piston scrapes back and forth across the seal. Eventually, the seal will fail and the caliper will leak.
Although a caliper may not be leaking when the brakes are relined, there is no guarantee how much longer the seals will remain leak free. Seals and pads usually wear at the same time, so it does not make much sense to fix one and not the other.
It is extra expense and effort, but why should your customer have to repeat a brake job in six months or a year when the caliper he should have rebuilt or replaced starts to leak and ruins the new pads you sold him?
In a sliding caliper, only one side of the caliper has an apply piston. The caliper moves in relation to the rotor and is held in a frame rigidly attached to the steering knuckle.
As linings wear, the piston gradually moves further out in the caliper bore as the pads wear. When the piston is shoved back in to accommodate new thicker pads, any dirt or corrosion on the piston will be forced under the seal and accelerate seal wear.
Another reason for rebuilding calipers is because rubber piston seals deteriorate with age. A piston seal performs a two-fold function; it seals the piston so hydraulic pressure can apply the brakes, and it helps retract the piston when the brakes are released.
As the piston is pushed out by the brake fluid, a square-cut seal twists slightly. This helps pull the piston back when the pressure is released, allowing pads to move away from the rotor more easily for reduced brake drag and improved pad wear and fuel economy.
Heat ages the seal. Over time, it loses elasticity and becomes brittle. This reduces its ability to deform and pull the piston back.
A neglected caliper can become a dragging caliper, causing increased pad wear, fuel consumption, and possibly a steering pull.
Rebuilding a caliper usually costs less than replacing it with a remanufactured or new unit, but it does involve extra time and effort.
Many professionals prefer the convenience of replacing old calipers with rebuilt units rather than rebuilding the calipers themselves. If a caliper can’t be rebuilt because of damage or severe wear, replacement is the only option.