Brake Caliper Piston Seals
With the exception of low drag brakes disc brakes have little clearance between the brake pad and the rotor. As the piston seal retracts, it pulls the piston away from the rotor. If the pad does not retract properly, the brake can drag, because the pad stays in contact with the rotor. This friction causes the rotor to overheat and deform. It will usually cause the vehicle to pull to the side that's dragging.
A restriction in the brake line will also cause brake drag. This is because the master cylinder creates enough pressure to bypass the kink, but the seal is not strong enough to bypass the restriction. This leaves the brake applied.
Brake caliper's have a dust boot to protect the piston and seal from road debris and water. Inspect these boots for wear each time the pads are replaced. Overhaul kits typically contain a new boot and seal. Square cut piston seals and other rubber components are sensitive to water, oil, and contaminated brake fluid. Always lubricate the piston seal with clean brake fluid or brake assembly fluid when rebuilding a brake caliper.