Even a new VCV isn't absolutely vacuum tight but in operation it should hold input vacuum, reducing to about zero with increasing throttle input. The purpose is to simulate gasser manifold vacuum. So engine with the engine idling, you should see as much vacuum through the green damper as entering the VCV. Hook up a vacuum pump or another car as a vacuum source to the VCV and a vacuum gauge downstream of the green damper. You should see full vacuum at idle tapering as you increase throttle. I'm guessing you don't want to stand on the throttle in your driveway.

Sixto
87 300D