Rear brake seize but only after seals changed

[Rubes]

Blew a seal the other day. Did a rebuild with new OEM seals. Caliper is seizing now. Doesn't release pressure if I crack open the bleed nipple so suggesting it is the calipers.

The pistons were very clean with no rust and just the usual crud around the exposed piston which was easily removed with a fingernail. The barrels had some light crud/ possibly rust around the extreme lip which I removed as much as I could with a fingernail. Some minor marking horizontally across the middle of the pistons but couldn't feel anything with my finger nail. I did the pistons one at a time so no possibility that I accidently swapped them over. Were working fine before the seal blew. Anyway when changing over the seals I noticed the pistons were very hard to get back in. Did eventually go but after much coaxing and a fair bit of force. Both fully retracted when refitted to the bike. Have removed pistons again to check I didn't foul the new seals and all looks ok. Get plenty of pressure when bled. Just doesn't retract at all. Basically I can turn the back wheel using both hands and some force so it isn't a case of normal dragging. Haven't ridden it yet.

Any thoughts?