For whatever its worth that little O-ring in the air inlet valve in your airbrush isn't available by itself, you have to buy the entire valve, which is relatively cheap. You have to be careful when cleaning the gun not to soak the gun such that the air valve is immersed in L-thinner. The O-ring swells up and stops the valve from moving properly. You can take it apart and just lay the little O-ring out in the sun for a couple of hours and it will shrink back down, but that fix only lasts for a couple of uses and it will start sticking again. The drying out thing just gives you some time to order a new air valve. The part number on the valve is "1 505 1", I think the spare one I have here cost me something like $12, but I really don't recall exactly.