You need a special tool that compresses the piston and then piston has to be rotated clockwise untill it is almost fully back flush in the caliper.If you cant borrow the tool ,take the caliper off and take it to your local Garage.For a small charge he will do it for you.Just done one.I take it you are putting new pads in?
Ray harper @ wybunbury@TISCALI DOT CO DOTUK., April 2008