[rnln]

On the "ball" and "port" and "hole" I mentioned above, on the manifold that comes down at the blue arrow, that port is for the air hose to connect to. Right at the elbow, facing downward is a hole. I think that is where the ball came out. I look through the port, don't see anything in there, so I think the ball just fitted in there. It doesn't look like that there is a valve there. It's just at that hole now. The ball is stopping air to come out at that hole. I try to push the ball in from the outside (bottom up), it fit tight. It can hold air for a while. When PSI comes up, the ball pop out again. I can try pressing it in, or dropping it from the inside through the port, but it can come out once, it'll comes out again, right? Question is, what does that hole/ball do, what is it's function?

Reading the article on the link from Mark Henry, I don't see it function as a check valve. I do see another check valve on another manifold, the valve with a key chain ring. You can see it on my third picture on the right on that port.