This is a repost of mine that may help
This question has me stumped as I thought the "Triplock" always had an O-ring on the exposed part of the stem where this Triplock as the gasket on the inside. Again I may be simply wrong in my reading of the Triplock diagram below but I don't see the O-ring in the doc.
Thanks
Bill
Source: Pictures are borrowed from the internet. No distinguishing marks so it looks OK.