The unit uses a connection to a compressed air source. The unit reduces the air pressure down to an exceptionally small rate and the ball in the tube measures the flow. If, after calibrated, there is no flow of air out of the test hose, then you can assume there is no leak. (The ball falls to the bottom.) If you can test each individual part and assure there are no leaks, (No flow), you might have either a faulty PCM or wiring to the NVLD.