something I hadn't heard of until now... when breaking free a seized engine, consider the aluminum oxide. Lubricant type rust penetrators, they won’t dissolve aluminum oxides

 Kroil, PB Blaster, and trans fluid will do nothing about aluminum oxide, and so, once you've soaked the cylinders in those, and it won't budge, try cleaning those out, and then giving white vinegar a try for the same amount of time

another new to me tip is to make a plate to fit over the cylinder and use a portapower to hydraulically push the piston, if all else fails

https://www.facebook.com/groups/496619540483459/permalink/4166880593457317

see  https://justacarguy.blogspot.com/2010/10/penetrating-oils-compared.html for a comparison of penetrating oils