Make sure the actual pump doesnt leak.. I made on similar to this for outdrive tests - its pointless if the pump leaks anywhere since a pump keeps the valve open when connected (of course you could pull it out quickly but that usually costs u a few psi - not good on lo pressure tests). I put my pump in a bucket of water pressurized and lo and behold - bubbles.
I bought a one way pressure activated valve on amazon for 5 bucks that I plumbed in after the schraeder valve - it makes sure air can only go one way - works like a charm!
http://a.co/2TPtijR