Don't try to plug that hole it is designed into the pump so when it leaks it won't kill the bearings too. Replace now along with a new belt and possibly a tensioner. Look closely at the belt routing before taking it off.!!

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