The vacuam lines do run through the rear floorboard, but I can't imagine that causing the problem you described.

BTW I dealt with back floorboard water after rainstorms for years. Finally thanks to this forum, I cleaned out the drain ports under the hoodhinges and just inboard from there with a piece of baling wire (for city people, a coathanger would probably suffice) and have yet to have reoccurrence of the problem.

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