bottle air vent help

[raj9000]

Should the screw cap on my bottle air vent be tightly screwed on, loose or removed?

 

[plumbnuts]

It should be undone a couple oof turns, you will notice that it has a small hole in the side of the cap. This is to let the air escape and as soon as water reaches the top of the valve it closes, therefore only allowing the escape of air.
Steve