I changed out the self levelling setup in my BMW 750i to a bilstein shock and spring combo and it was really easy to do. If the Mercedes system is similar, then you need to plug the accumulator outputs, and remove the link rod from the anti-roll bar to the levelling valve, and lock the valve in the fully 'up' position. This will allow the fluid to continue to circulate, but none to reach the accumulators. I guess the plugs are there just in case.
I believe the accumulators used are the same as in my BMW, I purchased the required plugs from a hydraulic specialist for a couple of pounds.

Hope this helps

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Ben

'85 500 SEL
'85 928S2
'88 560 SEC
'90 750i