Usually it's the copper seals around the fittings of the fuel pump(s) that are at fault. I have seen very rarely where the large o-ring that seals the pump casing does infact leak. In this case you do have to install a new pump...Bottom line, you have to trust their judgement or get a second opinion.

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