Almost invariably, the first component to go south on these vac systems is the vacuum bellows assembly in 3 of the doors, the fuel door, and the trunk lock. The driver's door has a "master" vacuum actuator near the handle, and yours may be bad. An easy replacement...buy it new here or get one from the boneyard. I happen to have an extra that I will send you for free if you determine that it's the problem component.

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Matt
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1995 E300 Diesel (Die Blau Frau)