According to the web site the system operates at 8000 PSI, incorporates a (proprietary?) "oil-to-oil piston type" hydraulic booster, and the cylinder uses "special aluminum alloy all metal ram seals".
There ain't a pressure bypass either... it will just continue to build pressure until it stops the engine cold if ya' can find a chunk of wood that tough... which is possible, I've done it a couple times.
You'll also notice there ain't an oil reservoir mounted on it... the beam is the reservoir, and there ain't a vent either. The oil level is checked and filled (I've never needed to add in over 30 years, and the oil is still crystal clear) by removing the large bolt above the handle at the wedge end of the beam.
There's a story (or rumor if ya' rather) that at one time they would sell the pump/cylinder/valve/booster setup separately if'n a fella' wanted to build his own splitter. Supposedly, as I've been told, they stopped doing that because it would bend or twist I-beams and W-beams, causing the slide to track out'a line thereby ruining the metal ram seals (shrug)
*