I have a 1991 560SEC. Though supposedly not required, I bled my system to replace the fluid. I did this by turning the car on, and opening the bleed valve back on the leveling valve down by the rear axle. I would stop periodically to refill the reservoir (under the hood) with new fluid. I did this until the fluid coming out of the bleed was clearer.
CAUTION: be careful when you open the bleed valve and don't crush your self. The back end of the car will drop quickly when you open the valve.
I bled this after changing the air cells. My ride was a little harsh, and the read end was bouncing like a car does with bad shocks. The air cells took care of the problem, it rides like a dream now.
John