They all work the same way. Most likely one of the diaphrams has developed a pinhole leak. I use a mityvac on each until I find it. Also, since you have the door panel off even if the one you test is OK it might make sense to replace the rubber diaphragms anyway to ensure you won't be facing trouble later.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz