I have a small (35gal) aux tank (used), but it doesn't have a vent valve; I think it is causing a fuel flow issue. I have it rigged for gravity feeding and I think it is creating a suck-vacume issue as fuel drains (not allowing air to come in to the tank as fuel drains). Does anyone know where I can get a valve for the tank?

 

Do a Google search for 'fuel tank vent valve' - lots of valves available. Have you considered using a pump instead of gravity feed?

 

the fuel cap should have a vent valve built in, or I believe if its a non-vented cap it should be marked as such

 

I used these place for parts when I built my tank.
http://www.southtownsllc.com/