Typically, door vacuum leaks occur where the vacuum hose runs between the body of the car and a door hinge. The frequent opening and closing of the doors causes the hose to crack at that point. This most often occurs on the driver side door as its the one that is opened and closed most often.

Remove the rubber tube between the body of the car and the door hinge at each the doors and examine the vacuum hose closely. Sometimes you can also hear the distinctive "Hiss" of vacuum loss. You will most likely find that its cracked there

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'95 E300 Diesel, 264,000 Miles. [Sold it]