I was taught many years ago that the cap is the primary seal. The valve is there as a convenient device to allow one to inflate the tyre and act as a secondary seal.
Was I led up the garden path all those years ago? Possibly, I'll have to dig out my copy of CAIPS, maybe it's in there.
It used to be a CAA oral question, How can you tell if the cap is servicable?
Answers on a post card........
