Sticking Brake Caliper Piston
Water contamination results in a pitted, rusty caliper piston and bore. Caliper pistons are typically phenolic resin or chromed steel. In time the protective chrome and nickel plating wear off a steel piston leaving the steel piston exposed. The piston bore becomes rusted and pitted from fluid contamination and dirt, resulting in a sticking or frozen brake caliper piston.
Each caliper unit contains a dust boot to protect it from road debris and water as the vehicle travels through puddles and dirt surfaces. Inspect these boots for wear when replacing the pads. Overhaul kits typically contain a new boot and seal.
A low drag brake caliper has a groove that's cut at a larger angle than a conventional caliper. A conventional caliper has a groove cut at around a 15° angle; a low drag system has a groove cut at a 30° angle. This design allows the seal to pull the piston further into the cylinder, providing less drag on the front brake calipers for greater fuel efficiency. These systems have become popular in many late-model vehicles.
Square cut piston seals, and other rubber components are sensitive to water, oil, and brake fluid contamination. Always lubricate the piston seal with brake fluid or brake assembly fluid when rebuilding a brake caliper.