its a floating caliper design. The slide pins could be rusted/gummed up preventing it from doing so. It could also be a stuck piston or a brake line deteriorating. Look for bulges in the brake line feeding it. They deteriorate from the inside and may have a flap come loose that acts as a one way valve for teh brake fluid and it will keep the piston from retracting. If its a stuck piston, replace the caliper. Rebuilding is just replacing the seal and dust cover and its a pain.