Replace slider pins.. When brake is applied, caliper piston pushes out against inner pad. This in turn pulls outer pad in against the rotor. Release pressure & piston recedes into caliper, allowing inner pad to move away from rotor. Outer pad does not move away due to slider pins hanging up. I seen this literaly 100's of times. Use ONLY synthetic grease, as regular grease will not stand up to the heat generated by braking. This info is also available from Raybestos Brake.