I may be late getting to this, but I just had the same problem about a week ago. Air pressure wouldn't budge the piston one bit. The exterior was badly rusted and pitted, but the inside wasn't as bad. The reason it was jammed was because the old brake fluid turned into a solid. Here is how I was able to get the piston out, and rebuild it.
First it was left to soak overnight in a acid based degreaser. It helped some, and made the caked up brake fluid on the outside look like a fungus.
Then the rear master was hooked up to a working front master cylinder, and the reservoir hole was plugged with a bolt.
After bleeding the air out of the lines, and building up pressure, you could push the lever as hard as possible, and then feel the piston move some under the pressure. Then build up the pressure again and repeat until it is far enough to pull out with pliers, or vice grips. You may need to add more fluid to the reservoir during the process. Look at that nasty junk!
The spring was still stuck, but it came out with some long needle pliers. To polish the bore I used a gun cleaning kit hooked up to a drill, because dremel bits were not long enough. It worked really well.
