If it is on the front, a c clamp, using the old pad or a block of wood to cover the piston will be helpful, doit slowly. On the rears, ther is a cross in the piston that you use to screw the piston back in, like with the end of some needle nose pliers or something. It is easier to do the brakes if you put the c clampon immediately after pulling the caliper, prior to the piston working its way out