Bad fuel cock? (video)

[Einsi]

Hi guys. I have a strange problem.. or maybe not a problem at all.
I have the bike running with the fuel tank laying on a table beside it (to adjust the carbs). Fuel cock set to Prime.
The tank is higher up than the bike, and more than half full, so the fuel should flow easily to the carbs.

There always seems to be an air bubble in the top of the fuel line that connects to cyl 3&4.
It moves into the fuel cock when I tilt the tank, and then comes back again.

Does this mean that the fuel cock is bad? Or am I missing something obvious and being stupid? :P
Or should I just be happy and don't worry because the bike runs?

A link to the video, hope it works:

https://www.dropbox.com/s/7efqltkxvtwacdq/Fuel.mp4?dl=0

Thanks guys.
Einsi

[RoXtarLeo7]

Maybe its even air getting in where the test lines are connected. Air bubbles would accumulate and move up to vent when you lift the tank. Then when the tanks down they build up again. Maybe get see-through fuel lines and see if the same thing is happening with a normal system. Air persistently wants to rise in fluid systems so its not likely coming down throu the cock