Along the lines of what Jim said, consider a large diameter air cylinder. Lots of area, some air pressure and you have a bunch of force on the rod.
Hook that rod to the rod of an opposed small bore hydraulic cylinder. Your large load from the air cylinder can produce a larger hydraulic pressure.
Give the hydraulic cylinder a simple check valve so when it strokes at high pressure, the fluid that's pumped out is stuck behind a check valve. When it goes back home it fills again from the reservoir, which can be done from another check valve that's opposed in direction to the first one.
The air cylinder can be made to "pump" or cycle continuously with a little valve that trips at the appropriate time to shift the pressure and exhaust ports on the air cylinder.
Clear as mud?