Found this
Here
Of course, this firm have an agenda, sell you a sophisticated valve cap, but it does clearly state......
It is generally considered that the tyre valve cap is intended to keep dirt and water out of the valve and protect the valve core which is the air seal. Although this is an important function of the cap, tyre manufacturers will tell us that the cap is acually the primary air seal for the valve. Valve cores can leak and it is therefore essential to have a proper cap with a seal in place at all times.
And also here
National-fleet.co.uk/authorisations/TyreTraining.
National Fleet.
The Dealerships route to Fleet Tyre Sales
Valves
Play an important part in maintaining the correct pressure within the tyre.
A valve cap in good condition is essential to provide a primary seal and prevent dirt getting in.
Ooh! And here...
Michelin.co.uk/tyres/learn-share/care-guide/ten-tyre-care-tips
The valve cap is also important. It’s the primary air seal and helps to keep out dust and dirt particles. You should check that your valves and valve caps are in good condition
I'll stick with what I was taught twentymumblemumble years ago.
