If your car has more than 60k on it, I would suggest replacing the P/S reservoir. It is about $25 and contains a non replacable filter. For years MBs had the P/S filter a replacement item on 30k services. The new arrangement starts causing problems after 100k. Even if it isn't your problem it is a strain on a relatively fragile pump (by previous standards) to not do it as a maintenance proceedure.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician