Before taking a gamble on a stuck piston, disconnect the P-brake cable, try to push the cable inward and pull out, if the cable will not slide within the housing, it has a kink. A stuck rear piston or P-brake cable won't result in a mushy pedal, that's another issue.
My master cylinder failed at the track a couple years ago. It was dripping fluid into the driver's front wheel well. Easiest way to diagnose is to turn the car on and stand on the brake pedal. It should go down and stop. If it drops all the way to the floor with continuous pressure, you probably have a leak. The master cylinder is located roughly between the intake and the windshield on the driver's side, silver-colored with thin pipes coming out. Leaks usually drip down into the well below.
Keep trying, there is a solution, you just have to find it. I thought my whole car was screwed up for 4 months, couldn't figure out why my pads were squealing so bad. Turns out I had a different type of brake pad on the right side of the car vs the left side, forgot to keep things straight when I was changing pads. You'll figure it out.
