Looking at the pics and thinking. Make a U shape of some scrap material that you can put into the tank and apply a lift from the underside, gentle and patience may work. What is the tank made of, Brass has it been soldered ? As paul has said boiling water or even hotter, if it’s a sealer it will eventually give up. What has stuck ? the linkage or is it the piston, or may be the valves ?
Mine had a perfect paint job – finding ALL the screws was the biggest task. In my case it was stuck valves. Good Luck. Noel.