Valve caps are designated "primary seal" in engineering exams

I'd be genuinely interested to see where that's written, propnut. There's nothing about it in the UK CAA Civil Aircraft Inspection Procedures. What they do state, is when tyres are being leak-checked after mounting on the wheels, the cap is not to be installed to allow the valve core serviceability to be proved. No mention of ever routinely checking the yellow 'primary' installed caps with leak detector spray. It would obviously be a waste of time if the valve core is working properly so how could it tested for serviceability? I've never seen caps being specifically called up for any maintenance other than general visual inspection. Considering some tyres are inflated to 200 psi it's hard to follow that a 'primary' seal wouldn't warrant any kind of function check ever.
This subject came up on Pprune about ten years ago and it turned into quite an entertaining thread.
See here: https://www.pprune.org/engineers-tec...p+primary+seal

Link didn't work for me. Quote it, presto, abracadabra, she goes good.

More edit: Now original post works for me too?!?!

MICHELIN AIRSTOP is what we have changed too after similar experiences on an RV-4 and RV-8 (so 5.00 x 5) wheels. It's a real PitA when there is a flat on these things - many screws to get the wheel pant off and if you try and drag it along with a flat it tends to damage the pant anyway.

We now bite the bullet and change the tube with every tyre change. This time (just done a tyre change) one of the tubes had been "welded" to the tyre at one spot. Carefull inspection makes me think that the tyre at some point had been locked (or scuffed - it is a taildragger :} ) and things got a bit hot..

You can feel the difference between the cheaper tubes and the Airstops. You do pay for it though, but we think it's worth the cost against the absolute b*ggeration of getting a flat.