Kaiser Sose formulated the question :
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?

The rattle is the float inside. They vent until the float rises on the
column of water, then seal against the outlet at the top. You can
remove the cap and prod the float down with small probe to check they
are not stuck.

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk