What series wagon do you have. Are you saying that you can only push the car down 2 inches, then it feels like it's hitting something? Also is the car running. I own a '83 TD and am not aware of a firmness setting that you can adjust. If the back end is bouncy then the problem is the accumulators.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member