I recently had brake fluid leaking from my rear.

I recently had brake fluid leaking from my rear passanger side caliper. Unable to tighten the screw to stop leakage I purchased a new brake caliper from Auto Zone. After replacing the caliper the leaking stoped. Soon after replacing the caliper the piston froze in the out position, so the brake are constantly being applied. I took the caliper back thinking that it may be defected, and got a replacement for that one. As soon as I put the newest caliper on and hit the brake, it to got stuck in the out position as well. I have since replaced the brake line to that caliper thinking that it may have been the problem. Even after tinkering, and replacing not one but two seprate calipers and a brake line my rear caliper is still froze. This has happened before about a year ago, that time I was unaware of the issue and continued to drive the car. After several months of it being stuck with the grinding and the smell my whole rear end fell off and had to be replaced. My question is What is it that is making my rear calipers freeze? And how do I fix it?

I have already took the piston and sliding pins out of the newest one to try and clean it, and still no luck! We checked the master cylinder as well, there is no leakage and the resvouir is still full, when brakes are used they ARE NOT soft and go to floor. They feel perfectly fine.

Any tips or tricks would be greatly appreciated! Thank you.