When is a puncture not a puncture?

[Deleted member 1258]

Some Presta valves (Specialized, for one) have removable valve cores. If these leak, I take the core out, put a wrap of PTFE tape on the thread and then screw it back in tightly.

A few years ago one of the tube makers, I'm sorry I can't remember which one, had a batch of presta valved tubes with screw in valve cores leave the factory with out the cores being tightened down hard, we were tightening the cores with pliers before we used them.

 

[KneesUp]

Thanks for all your advice and suggestions. It's a proper leak now so I'll take the tyre off tomorrow and investigate.

The tyre was only at about 40psi (it's 38c) but my track pump arrived today so I put it up to 70psi and the air was audibly coming out from near the valve, I think from inside the rim though, where the valve meets the tube, so I guess it was a small hole that turned into a bigger hole when the pressure went up.