In order to ensure that your brakes are functioning properly, it is important to know how far the caliper piston should retract. The answer may vary depending on the make and model of your vehicle, but in general, the piston should retract about 1/8-1/4 inch. If the piston does not retract this far, it could be a sign that there is something wrong with your brake system and you should have it checked by a professional.