Hi, you dont say what sort of car, or front . rear, might get more answers if this is added, for instance, Vauxhall rear calipers need to be pushed in and turned with a special tool, but ford front just push in (although you will need to apply a lot of pressure), best way i have found is bolt the caliper back on, then use a large screw driver or crow bar to force the piston back in. Hope this helps
Sussex_Chris
May 2009