if its like a chevy I've done some of those. I take a small piece of wood and put in between the piston and outer caliper, and where the brake line goes into the caliper shoot it with compressed air, and it will pop out. you have to make sure the piston surface isn't rusted, pitted etc, and the inside of the caliper also. install new oring and reassemble. Its pretty easy, but still Your better off and time and money ahead to just replace the caliper though. I always replaced them in a set, both fronts at the same time.