Don't ever pressurize a cylinder with a grease gun or high-pressure hydraulic pump to try and loosen a stuck piston!
If you've worked on gas engines, you've seen cracked cylinders, cracked hoppers, and cracked heads. Almost all of these were most likely broken by pressure, pressure from the expansion of freezing water. Ice expands with great force when it freezes, and most engine people know enough to drain all water when temperatures drop. Hydraulic pressure inside the cylinder will act just like freezing water!
Cast iron is extremely strong in compression with ultimate strength over 100,000 psi, but it is extremely weak in tension with strength as low as 7000 psi. The old cast iron may have pockets of corrosion that have had seventy years or more to work, and these can reduce the tensile strength further. When you pressurize the cylinder internally, you produce radial and longitudinal tensile stress, and relatively low pressure will burst the thin cast iron wall.
What is the best way to loosen a stuck piston? Nothing beats the 'Red Wrench,' a torch! Cast iron can take tremendous amounts of heat without cracking or distorting, that's why old furnaces were made of cast iron. Heat will soften the varnish created when the old oil oxidized, and heat will break down and weaken the rust between piston and cylinder. So, if several days of soaking in penetrating oil doesn't loosen a stuck piston, consider using heat. Have patience, heat things slowly and heat the entire hopper and cylinder.
When you pound on the piston, never hit the center; the top of those old pistons aren't very thick. Make an aluminum, steel, or hardwood slug turned on a lathe just slightly smaller than the bore. Relieve the center so that it hits only on the outside edge of the piston. Heat and a few good licks with a sledge will get most stuck pistons' attention right away!