My passenger side caliper works fine with regular brakes, activates and releases.

However, if I pull the parking brake, the piston extends and brakes are applied, but the piston will not retract after releasing the parking brake. I am very confident the issue is with the caliper as I have gone as far as removing the parking brake cable after lowering the handle, and the brake stays locked on. The parking brake lever does not go all the way back down and I can't force it to (even with a hammer). It only returns all the way when I pull the caliper off and the piston is able to shoot out a bit more.

Worth noting that I purchased these calibers (rebuilt) just a few months ago. Ebrake cables are also new.

Any help or ideas appreciated.

Moved from "General GM Chat" to "Pontiac" for visibility.

Sounds like a bad rebuild to me. If it were older, like it had been on the car for a million years and has a million miles on it I'd ask about rust and damage etc. but this one is a recent rebuild and refuses to work with all possible contributors removed so it'* probably just bad.

Your rebuilt caliper is bad.

Bought a new caliper, again, and that fixed that problem.

Great news!

Thanks for letting us know!

Had both off my 2006 Grand Prix GT do that 2 years ago, I heard if you do not use the emergency brake once and a while the calipers will lock up..

Glad you got it fixed!