" The valve core is the primary, as has been said, and [B]then there is an indication within the valve cap that will tell you that the primary core is leaking before very much air/nitrogen escapes."[/B
Which usually comes in the form of a "psst" sound when you remove the valve cap....strangely enough.
However, you may wish to drop HM Gov't a line and politely inform them that, for many years, the training syllabus has been incorrect and that myself, and many others, have been labouring under a misconception as to what is defined as the primary seal....