Well the op titled this thread as a Master cylinder problem then proceeded to talk about calipers, So I had to ask which he meant.
Now with the more info he supplied we know he has issues with both.
Gas won't help much. A penetrating fluid will. About the best is a 50/50 mix of acetone and ATF, both available at parts stores.
Use a large c-clamp to push the piston into the caliper, this loosens the grip of the rust.
Now you can use air pressure to push the piston out, just be careful, when the piston comes out it comes very fast. Don't have your fingers in the way. A shop rag help stop it from bouncing off the wall.
On the master cylinder I might just replace it.
The old stock lines rot from the inside out. New rubber lines are better than the old. Braided stainless steel lines are even better. They don't flex under braking so more of your lever effort goes to braking.
Leo