Generally speaking, unless there is a check valve on that vacuum line, there should only be vacuum present when the engine is running. Since the source of vacuum is the suction of intake air caused by the downward stroke of the pistons, when the pistons are no longer stroking, they are no longer sucking in air. When the engine is not running, a vacuum gauge should show zero vacuum or atmospheric pressure.
If there is a check valve on that line, it may be faulty or it has been turned around backwards. There could also be a leak in vacuum lines inside the car that control the AC/heater vents.
1990 LE -wrong bumper