The factory check valves suck pretty bad. I havent seen one yet that seals properly. I made my own for the charcoal canister out of brass pneumatic fittings, with a little spring and ball bearing...its kind of like a miniature RJC boost controller, only its not adjustable. Its only about 3/4" long...really tiny but it works great. The more pressure it sees, the better it seals. You could probably call up Clippard, ask for the tech dept and ask them what they have in terms of small check valves. Just tell them its a custom automotive application and tell them what you need it for. Even McMaster.com might carry something you can use.