Guys,
How do these bottle air vents work? Tried to search on the web and can
find nothing. I have just bought one and when I shake it something solid
rattles inside. I am replacing the one that is seized up and sits at the
top of a piece of pipe in my airing cupboard. Does the rattling bit press
against something internally when full of water?

It has 3/8 connection and a sort of twist valve at the top (like the top
of a biro pen). Should that twist valve bit be screwed tight or loose?

Thanks, Steve