Ytell,

The vent line is just a simple tube it does not have a one way valve.
When the tank is full, the overflow comes through the vent line and out into your catch bottle!
You simply position a catch bottle under the angled brass tube during filling. A 12 oz bottle will just slide up over the tube, which holds the bottle still, ready to catch the overflow. It's the most conveinient system i have ever seen

The vent line is just a tube run to the top of the tank on one side of the tube, and the other side of the tube is the angled brass tube you see in the picture...

You may be wondering why the fuel does not pour out when you are inverted. It does not pour out because the tank is not pressurized as in a regular pressurized system. Also the angled brass tube is bent in such a way that when inverted, the end of the tube is above the fuel line. Pretty cool eh? My own design!

Make sense?

The fill and engine pickup are run to clunks in the tank. This is call a 3-line system.