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Try using a grease gun. It's messy, but it will work: https://www.musclecarresearch.com/grease-gun-pressureYou can also heat the block with a propane torch to soften the seals before applying compressed air. Just block the open end with a shop rag or something similar. When it pops out, it will come out with a lot of force.
+1 You will find this more effective over just using air but good to try that first. Others might soak it first to see if that will help the piston free up but the fitting and grease gun often works. Challenge it finding a place where the piston will not rocket off some where and be a challenge to find or damage it or something else in the process. You'll come up with a solution. All depends on hat you have around the shop
Unfortunately compressed air did not work like it did on the proportioning valve. It seems to be really seized in there. However it does move with a punch.
Guessing that you have soaked it for a day or so in an oil or brake fluid.