The tool you need is available at most auto parts stores. They make a universal model that fits on a 3/8" ratchet. It is cube shaped, and has a variety of knobs that will fit into the detents on the end of the caliper piston. The idea is to turn the piston as you push it back in. The old method of squeezing it back while releasing fluid pressure will not work and will probably end up damaging the caliper. If you cannot find the tool, three hands will be required. Hold the caliper, while applying light pressure to the center of the piston. At the same time, use channel-locks to turn the piston clockwise. Be very careful not to score the ends. Suprisingly little pressure is required to get the piston back in.