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During a recent ride my tire went flat on my tri bike. I was about to change the tube but thought I'd just put some air in the flat tire first. Amazingly it held air for the rest of my ride (another 2 hours). Why does this happen?

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    Possibly something in the valve, which then got cleared. Apr 26, 2020 at 18:52
  • The valve core wasn't fully tightened. Pumping the tyre may have done that without you noticing. Get a core tool and retighten the core on both valves. Sometime these tools are packaged with new tubes. You can achieve the same by gently applying pliers to both flats on the top threading of the valve (the threads that hold the valve cap).
    – Carel
    Apr 26, 2020 at 19:12
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    Are you sure its tubed? Bikes with tubeless tyres can "burp" air and then recover their seal. Could also have had sealant inside the tube which eventually did its job. Good work for having a spare tube, pump, and tools!
    – Criggie
    Apr 27, 2020 at 4:55

2 Answers 2

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When I was a child I learned that with a flat you would first pump up the tube and wait overnight (if you had the time) before going through the effort of patching it.
As part of that pump-up action you would check the valve was tight.

Only if flat or soft again the next day we would spend the time and effort patching.

Very often it is indeed the valve which has a problem. Different types of valves can have different problems, in the ones we used there is a ring pushing the actual valve in position and it is rather likely for that ring to be less than tight. And those valves we used when I was a child had a bit of rubber tube, which often failed, so it was not uncommon to take the valve out and replace that tube. These days I often replace the current valve with a new one, just to make sure.

Other kinds of valve have a different system of closure and those are really likely to be tightened properly when you finish pumping or may re-set themselves when the tube comes to full power.

Slow leaks are often slow enough to hold air for quite a while. Do check the outside of the tire if you want to keep riding after a flat, as if a sharp item is logged in the tire, it will make for a much worse set of holes in the tube. There are different approaches. If you will replace the tube anyhow, you may want to leave the hole-maker in, to plug the hole. But if you need to patch the tube, do not ride any farther with a sharp in the tire.

I own a bike cart which has been allowed to have a slow leak for more than a decade, more likely closer to 25 years, as the loss of air was so slow that we would never be surprised by a flat while underway. I have now a new tube for it, as it is now no longer a slow leak, it was flat in about 10 minutes. (But I have to admit that I have not checked the valve yet.)

So yes, if you have the time, first just pump (and check/properly close the valve) when you have a flat.

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This is happened to me before. Sounds like a valve and/or pump issue. I had this happen to me where I'd inflate my tires and ride only to have the tube lose air with no holes in the tube. After getting a better pump the problem went away.

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    How would a pump make tire go flat while riding, after it has been detached the valve?
    – ojs
    Feb 3, 2021 at 15:50
  • Did the old pump have a screw-on chuck? It may have been unscrewing the valve core a little when you removed it. I once dealt with a guy on the side of the road whose valve core would be removed before the pump head came off. I had to use my CO2 inflator for him.
    – MaplePanda
    Mar 6, 2021 at 16:20

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