Depending on what actuates the mechanical parking brake inside the caliper, a series of ratcheting should do the trick. Usually there is a bendix (which is why you have to screw the piston back into the caliper to install new pads) inside the piston that allows the parking brake to mechanically index the piston when the brake lever is pulled, released and pulled. IIRC Some times it helps to hold the brake pedal down to insure that the piston is tight against the rotor.
