You might get lucky with just blasting it with PB Blaster or Kroil. If that doesn't work, then you're going to have to pull the pedal assembly out and drive it apart and clean the crud out of the pivot and use some good waterproof grease, like boat trailer wheel bearing grease, on all the moving parts. Once you can actually get some movement on the master cylinder piston, you might get it to bleed. Can you see the entire brake line? Is it OK, or is it smashed flat or pinched?
Other thing is when you get it freed up, don't push the pedal all the way down. Most of the time there is rust and other trash at the end of the master cylinder bore; the rubber cups don't get that far when everything is working right. But when you open the bleeder, now you can run the cups into that and cut them up. Don't know if you can get the rubber bits or only a new master, which I'm sure costs a fortune.
I had a frozen throttle pedal on my Ford truck, tried the easy way and no joy. So out it came.
