Anyway to fix a seized up caliper? Had to buy 2 new ones for the front and kept the old ones laying around. One of them has a seized up piston. If I can fix it, I'll keep them as spares or maybe even sell.

 

You can soak it in WD40, but if it's seized, the bore is usually damaged.

 

WD-40 is a water-displacing lubricant oil. To separate seized parts, use a penetrating oil instead, or a nasty petroleum-based solvent (depending whether it's rusted or gummed up). I'd say soak in penetrating oil for a week (not like you're in a hurry right), tapping hard with a mallet a few times every day after work. Then try to separate the parts.



I'm betting there will be pitting or worse, and it will have been a learning exercise but without a new set of pistons (at least) there are good odds the calipers are toast.