The 3-way valve in the driver's door. Its called the master switch or something like that, but its a 3-way valve.

If your doors are locked, then that solid yellow line goes to the 3-way valve, and then out that valve to the yellow/red lines. So if you checked all your yellow/red lines that lead to the other doors, then the 3-way valve is leaking. If the doors are unlocked, then the above is true for the yellow/green lines.

To check the 3-way valve without taking the door panel off, pull up the drivers carpet and get to the vacuum lines. Pull the yellow/red line that leads to the valve in the drivers door, and plug it. Lock the drivers door and draw a vacuum on the solid yellow line under the hood (or plug that one and pull the vacuum on the yellow/red line, whichever you want). Repeat for the yellow/green line with the door unlocked.

If its not that, it could be any of those black rubber connectors.

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1985 300TD-euro 352,000 mi
1974 240D (1?)52,000 mi - has a new home now