You know you have low vacuum available. Pump up your mitivac to the vacuum you have measured while hooked up to the shut off. It probably will not shutoff.

Now increase the vacuum until it does. That is the value you have to provide or exceed. With your Mitivac for it to work. You should be able to find out why you lack the vacuum to that point if you do.

Four pounds of vaccum is really not very much. If you orally suck on a vacuum gauge you can easily exceed that amount. Actually it sounds like you have been a little confused. The vacuum line to the part you want to test goes to the injection pump. If it will not shut off or hold a vacuum if the part is bad.