Rear Brake Caliper Piston will not compress?

So, what happens sometimes is the piston will come undone from the parking brake screw internally. I would apply more pressure will you turn.

Please go over this video to help you fix the problem:

https://www.youtube.com/watch?v=j8YwfSKEzZo

Check out the images (below). Please upload pictures or videos in your response of any problems so we can see what to help you with.

You need to compress and twist at the same time. Being a 2008 the seals are likely stiff. It sounds like you are trying to use one of the ice cube tools. You will need to get the type of tool in the image. They can be found as a "loaner tool" at many parts stores, you basically buy the tool, then when you return it in good shape, they give you back your money. Then you use that tool to apply pressure while turning. There were a few years when there were left hand thread parking brake calipers as well. I don't believe that one has those though.

When you removed it there was still pad material left, correct? Did anyone push on the brakes when the caliper was off? Normally the only time there is an issue would be if the piston extended too far and the threads in the rear disengaged. That can happen if the caliper was over extended. If neither of those occurred, then the next likely item would be a bad flexible brake hose. To see if that is the problem, apply some pressure and loosen the bleeder valve on the caliper. If it now compresses properly the hose is likely bad inside and acting like a check valve. If that is the case, you replace the brake hose and bleed the brakes with new fluid.

https://www.2carpros.com/articles/how-to-bleed-or-flush-a-car-brake-system