The piston is hollow! Look at it, you should see the caliper housing, then a spherical rubber seal, followed by the spherical caliper. If the caliper is stuck out, compress them back in using a c-clamp, then step lightly on the brake pedal again but do not go to the floor, just enough to note movement. You will see the caliper slide back out. If there were no calipers there, you would not have any brakes and you would have wrecked a long time ago!