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.
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