My mechanic noted a retention spring that goes around the hydraulic oil reservoir, and that it was sufficient for the oil to be above that level. The system should self-bleed, though I believe there is a bleed screw on the leveling valve. At idle, it is pumped so slowly, you can re-direct the dirty oil to a waste container as you fill with new fluid. I had the filter changed when I bought the car, but all the records show is the hydraulic fluid being changed every 15k.
Cheers, Andrew Seidel.