DO NOT use a dremel to clean the rust off of that thing. if you take too much material off of the piston it will not pressurize properly. just use light sandpaper to take that surface rust off. to reinstall the piston you have to install the seal onto the caliper, apply brake fluid onto the seal and the piston... and then use a wood block or old brake pad and c-clamp to push the piston back in. i'd also suggest new seals if you haven't bought some already.