You need to remove the brake pads from the caliper, and clean them up carefully, paying particular attention to the edges that come into contact with the caliper. At the same time, press the caliper piston back in with a 'G' cramp and see if its free. Grease any contact areas with 'coppaslip' grease or similar, and at the same time apply the stuff to the back of the pads.
John, April 2008
