On my 1997 E320, I have the same problem and I removed the trunk lining and I discovered that a vaccum nipple actually broke and I haven't yet fixed it because I don't have time. I tried using tubing to fix the problem but it still leaks. I attached a pic of what I think is broken. On the W210, the part is on the lid. On the W140, I think it is on the trunk deck, so accesibility is harder. I don't think you have to check your vaccum pump because it is easy to hear the hissing from the trunk tumbler.

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