I had that same problem with mine. I used a ton of penetrating fluid and hammered away until three of the four (two pins, two sides) finally came out. Still, one of them was stuck. I drilled off the head of that pin so I could hammer it out the "wrong" way. I would probably use a dremel instead of a hand drill if you have one. Anyway, that was the only way I could hammer at it with enough force to get it out (without removing the entire assembly).
Either way, I recommend buying new pins and putting some anti-seize on them to reduce your problems next time. Oh, and be careful not to get penetrating fluid on the rubber boots that surround the brake pistons. I did that and one of mine is a little "soggy" now. Brakes work fine though.