The valve can leak internally and can be somewhat intermittant in nature (IE sticky) and when warm act differently than when cold. The way to adjust it is to pull off the little cap on the front of it and slide the key ou to the top of it very slightly. Turn in at 90 degree increments to firm the shifts and out or counterclockwise to soften.

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Benzmac:
ASE CERTIFIED MASTER AUTO TECHNICIAN
LEAD TECHNICIAN FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM