Hi. The piston in the caliper has to be pushed back with a special tool. Thus turns and pushes the piston at the same time. There are 4 cut-outs on the piston face, these must be aligned with pins on the tool, otherwise it may jam. A tip here is to put a small amount of brake fluid on the surface between the piston and the seal that surrounds it. Keep an eye on the seal as you screw in the piston, it can pick up and split, so take care! You may be able to hire the tool, or purchase one from your local factors. Dont forget to pump the brake pedal before moving off!!
Paul, January 2008