I had the same exact problem with my first Benz, a 1980 300SD. The day after I bought the car, she started to roll back on me. I thought great, there goes the tranny. As it turned out it was just the bushing in the linkage which my mechanic sold me for $1.50.

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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