Well also if the pistons are not sealing correctly then they will not push out and consequently not push on the brake pads. But then again, I would assume there would be some brake fluid leaking. I had to replace the used ones I bought too, I got my new ones at cskauto.com. (checker auto parts) Anything from the parts store is going to be refurbished but there is nothing wrong with that IMO. The deal I got was two calipers, loaded ( that includes the caliper, mounting bracket, and brake pads) for 60 bucks apiece, plus a 50 dollar each core charge. They come all assembled and ready to bolt on. This might not be a bad option if you are trully having problems. The rebuild kits run about 20 bucks and only replaces the rubber parts on the piston, not the piston itself. By the time you bougth two kits for each caliper thats 40 bucks each caliper, you might as well get new calipers. Sorry for the length, just trying to help.