The disc brake caliper is the assembly that holds the brake pads and contains a hydraulic piston. Mounted over the disc brake rotor, the piston pushes the brake pads until contact with the rotor is made. Force from the caliper applies friction to slow and stop the vehicle. There are a couple factors that can cause a caliper to fail, including dirt, corrosion, piston seal failure, and infrequent use. These can cause the pistons to lock, keeping the brake pads rubbing when the brakes are not engaged resulting in excessive brake pad wear and extreme heat, and warpage. When replacing your calipers, it is best to check rotors for damage, and inspect the brake pad wear and thickness. When bleeding, always use brake fluid from an unopened container, as simply being exposed to air will cause it to go bad.
