When calipers do their thing during the course of a standard drive, a huge amount of heat is generated. This heat radiates directly from the pads and rotors into caliper assemblies, where temperatures can reach over 300 degrees Fahrenheit.
Over countless heat-up and cool-down cycles, corrosion can form inside and outside of the caliper. This corrosion comes from many places: the moisture in the air, whether it's humidity or precipitation; and any moisture that may have been absorbed by the brake fluid, especially if it hasn't been changed at specified intervals (see our related article. to learn more about this subject).
Because temperatures inside a brake caliper exceed 212 degrees Fahrenheit, any water that's found its way into the brake fluid boils and turns to steam. Countless repeated cycles of water boiling, then condensing, then boiling again is what eventually gives corrosion a foothold.
Corrosion which forms on the caliper pistons or cylinder walls is likely to create rough surfaces that cause abrasive wear on seals as the pistons slide out and back during operation. Because the piston seals also dry up and harden with age, they are no longer flexible enough to withstand normal forces or scraping - so they start to leak.
If corrosion reaches a critical point inside a brake caliper, it may seize up and not move at all if pistons become stuck inside their cylinder chambers. The problem can be exacerbated by pads which have worn almost down to the backing plates, requiring the pistons to extend even further out of their caliper bores.