The shop can probably do better adding some UV dye to the system and looking for a leak. This way they not only find out that there is a leak, but where it is.

Pulling a vacuum, then standing back to see if it holds the vacuum is only good for a gross leak check.

Good luck,

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Larry Bible
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles