I found an excellent source of information on maintaining the hydraulic suspension of my 1990 300TE. Also relevant for 560 SEL/SEC cars.
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Register with them, then search the forums for:
"hydraulic suspension" or "suspension fluid" - great info on how to perform fluid change, and save yourself some $$$ and hassle using the methods they talk about
"accumulators" - great info on how to replace accumulators, where they are in the car, etc.