rear brake calipers

Try bleeding that line a bit more while still applying pressure with the C clamp. Also the caliper piston may be stuck and you will need to remove it to fix the problem. This happened to me once some time ago.

You can not compress that brake piston with a c clamp you need a special tool that compresses and spins piston at the same time. You can rent this tool at autozone.

Thanks, I found a similar answer on a you tube video and was able to screw the pistols on the calipers in using a pair of need nose pliers.

The rear brake cylinder on my 2015 Hyundai Elantra won't spin or compress. Would releasing the emergency brake help ?

Thanks for the help. I had mine all apart before learning of this, so I couldn't go driving to rent the tool. Didn't get very far with "needle-nose" vise-grips (which might have worked had the piston been looser). Made my own tool by grinding a 20mm socket.