Jeff,
Hyd. lift was a factory option on the 1250 for sure, maybe the 109/129, can't remember. Doesn't make a difference though, like Ken said, it will be fine on the 12hp.
The hydro is constantly pumping oil throught the charge circuit at 180-200 psig. When you engage the lift circuit, the implement lift relief overrides the std charge circuit relief and the max pressure goes up to whatever it takes to lift the load or the relief valve set point (600-700 psig) whichever comes first. When the cylinder "dead heads" the pressure in the system is equal to the relief valve setting (600-700 psig). This is, by the way, why some older hydros will surge forward when the hyd lift dead heads...the higher than normal charge pressure leaks into the hydro drive circuit and "turbo boosts" the drive pump pressure.