Pull off the center vacuum line on the switch, which is the same solid yellow line from under the hood. It might help to also remove the line closer to the metal end of the switch, for easier access.
Use a suitable object, such as a golf tee, bolt, or large nail, and plug off the end of the solid yellow line that is inside the door.
Return to your still connected vacuum gauge under the hood and pump up 10 to 15Hg of vacuum. If there is a leak at this point, you know the solid yellow line that goes from under the hood in to the main switch is cracked. Make note of the result and continue to the next step.