I am going off the parts book, never been in one.
The pistons are sort of like a dumbell, should be near flat on the end you see when you pull the cap off. They are hooked together by a centre part with a hump in it, a link goes from this to the crank handle. The pistons assembly I would think is cast so be careful. There may be a flapper or valve on the piston face, would make sense.
Clean out what you can in the bores and try to get oil in the middle part, penetrating oil should work but any oil is better than none. You should be able to pound on the piston with a 2x4 or 3x3 using a framing hammer or light ball pein, just tap from one side, them the other, keep oiling.Try tapping before you haul out a torch.
Are the end plates good?, many are brazed up from freezing. Are you trying to get it to feel like it works, or actually use it??