You can use brake parts cleaner is what I use, I like the non flammable type. The piston won't come out as long as you don't push the rear brake peddle. Only remove one side and clean it at a time and if you push the piston in be sure to pump the brake peddle after you have installed the caliper, you could push to much brake fluid back into the master cylinder and cause it to over flow if you push both plungers in without pumping the brake peddle.