I don't know how much was changed on the self-leveling suspension from the W124 to the W210. My comments are based on my experience with the W210 wagon.

The system is a closed loop. The PS pump recirculates the fluid thru the PS system to the back of the car where a pair of accumulators supply fluid to the rear shocks as needed for height adjustment. The fluid returns to the reservoir thru a sediment filter located under the cap.

The part number on the cap is probably discontinued due to age. There will be an updated part number. Check with a dealer.

When I bought my car 2 years ago I could find very little information on the system and no service recommendations from MB. I siphoned the reservoir dry, filled with fresh fluid and replaced the filter. Some have suggested disconnecting the return line and using the system to flush itself. However, the fluid in the accumulators and shocks can't be removed without some disassembly of those components.

Hope this helps some. Good luck.