Vacuum is supplied to the central port of the master switch (by the yellow line).

To test the switch:

Sit in the driver's seat and close the door.
Plug the first (aft or left) unlocking port (yellow and green line), and the plug third (fore or right) locking port (yellow and red line).
Apply vacuum to the central port with the door unlocked.
Then, apply vacuum to the central port with the door locked.

If it holds vacuum in both positions (locked and unlocked) the master switch is okay.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022