That is call reaction valve for B1 brake band you need to replace two seals part number are 005-997-80-48 and 016-997-34-48 and 140-277-01-51 pin holder after unscrew the plug you can pull out the reaction valve replace seal with new seal and new pin holder when you put the valve back make sure the pin is in the hole of B1 brake band.

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MB STARS Master Guild Technician,17 years MB tech
14 years independent shop owner