Bottle Valve ?

[Maxx470]

I have bottle vent valves on both the hot water cylinder & the boiler.
On top of these are what looks like a car tyre dust cap.
I have had to drain the system down and on refilling it should I remove the dust caps ?. I did and air , then water came out. The water only really stopped fully when the caps were put back on. is this correct ?.

 

[seco services]

its sticking inside. should only let air out.