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I was doing my usual 15 mile round, about mile or two to go, when suddenly I realised my front tyre had gone flat. It has heavy duty, mountain bike style, valves. I haven't been able to find a puncture, re-inflated the tyre and so far it's holding 40psi. I rode it for a couple of miles without any problems. Could it be that the knurled collar came loose?

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  • Normally, I would say no, this should not be possible. However, this exact issue happened to my friend recently. I examined the tube under water to see if there was a micro puncture. I did not detect any. That would be the more likely explanation in your case, but I really did examine my friend's tube carefully in the sink. The knurled collar around the stem valve is something that I haven't found it to be useful at all, except on tubeless setups, and it should not have caused your issue. I'm leaving this as a comment for now, in case someone can give an alternate explanation.
    – Weiwen Ng
    Commented Aug 31, 2020 at 17:53
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    Is your wheel tubed or tubeless ?
    – Criggie
    Commented Aug 31, 2020 at 19:55
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    "heavy duty, mountain bike style" would normally be used to describe Schrader valves. But Knurled Collar implies Presta. What valve type do you have?
    – mattnz
    Commented Aug 31, 2020 at 22:24
  • Did you have the little knobbie thing screwed down? It's easy for, say, a twig to briefly get in the wheel and press in the valve of a Presta, if the knobbie isn't screwed down and the cap is absent. Commented Sep 1, 2020 at 0:22
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    Richard, would you be so kind to take a picture of your valves and post them here? — What happened to you is unusual for Presta valves but common for Dunlop and not all that rare in Schrader.
    – gschenk
    Commented Sep 1, 2020 at 14:39

3 Answers 3

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Possibly the valve was contaminated with a particle or dirt and was not sealing properly leading to quick but not instantaneous loss of pressure. When re-inflated the contamination was dislodged and the tube holds pressure once again.

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Presta valves sometimes have a removable core which sometimes get a little loose from screwing and unscrewing pumps. If not tight enough, air can escape.

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  • Schrader valves also have a core that is removable. If it was a little loose, same thing could happen. Welcome to the site - please take a moment to browse the tour and learn how the Q&A format works, but you're off to a great start.
    – Criggie
    Commented Jan 14, 2023 at 4:50
  • The removable core does not work well for high pressure, as the seal will not hold regardless of tightening. The Schrader valves are normally not used for tires of high pressure, so it seldom becomes an issue. In short: do not use Presta valves with removable core. Commented Jan 14, 2023 at 12:36
  • @NielsTolstrup In short: do not use Presta valves with removable core. Huh?!?! Conti tubes with removable Presta cores work just fine. Commented Jan 14, 2023 at 17:19
  • I cary a core tightening tool. It’s tiny and cheap and also works on shrader valves. It’s surprising how often it get used. If tires that tend to lose a bit of air over a few days, that often a core issue. I also had Presta valves cores unscrewing completely using a screwing pump. So why bother with removable cores? It could be removed to add sealant in your tires without removing them. Also if you break the little screw that seals the valves, you can replace just the core. Tubeless setup with no spare? Don’t break that pin! Carrying spare cores is also a good idea. I take mines off old tubes. Commented Jan 19, 2023 at 19:30
  • I pumped several new Schwalbe tubes with Presta valves to 120psi and found repeatedly that the removable cores could not seal sufficiently. I therefore hold that for riders that do not need removable cores and would like to pump to 120psi, Schwalbe tubes with Presta valves with removable cores are not recommendable. For lower pressures it is most likely a non-issue, but still, do I need yet another point of failure on my bike? Commented Jan 21, 2023 at 18:24
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Some (not obvious) problems with tires lead to very subtle punctures in the tubes. These punctures can only be found when the tube is inflated outside the tire and then checked either under water or by trying to feel the air going out by hand. They are not seen on deflated tube. When mounted, such a tube/tire holds the pressure but would spontaneously deflate at some point within a week or about.

Once I was able to locate the puncture - a very thin 4 mm cut in the outer side. It was a very easy tire/rim combination with all mounting done with plastic levers, so I do not believe I damaged the tube or tire myself, and this tire deflated multiple new tubes before I was forced to replace it.

Hence the valve may not be the problem, and if you screw it closed (you only need to screw it open to put the air in) and put the plastic cap properly it is unlikely to be seriously contaminated.

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