I've got a Shaver pounder.... no piston seals, kind of a star piston, like a sprocket. They're built that way so that the cylinder rod can drop fast/freefall when you let off the pressure at the valve. Piston and rod just fall down THROUGH the oil in the cylinder then, instead of all of the oil having to be evacuated out of the cylinder. Not much oil actually moving through the cylinder then, just pressure, pressure relief. It's pure "pressure" in the entire cylinder length then that drives the rod back up, not a "sealed chamber" on the bottom side of the piston head. Don't need alot of volume of oil, but pressure is important.