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I rode my bike 2 days in a row, left it home the next day and the rear tire went flat by that evening. I inflated it last night to check for leaks or punctures but it's still inflated, I always check my tires before riding and I had no problems during the two days I rode. The tire went flat while it was not in use and the pressure is still the same since I last inflated it. I don't see any visible reason why it would deflate so much so fast especially since it seems fine now. I'm concerned if I need to replace the innertube or if this was just a fluke.

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  • Perhaps relevant? bicycles.stackexchange.com/q/23647/6852 Aug 31 at 14:27
  • I'm currently dealing with a tyre that's losing about 1-2 bar/day; I just haven't had the energy to take it apart and find the leak yet. It's definitely leaking somewhere, but it's also fine for a 4-hour ride if I pump up before I leave. Your tyre/tube is definitely leaking somewhere, and it will continue to do so. Depending on the tyre size and what pressure you run, you may not notice in a hand check if it's down 20%, so it may seem to suddenly have lost pressure when you finally notice it dropping from 70% to 50%.
    – DavidW
    Aug 31 at 14:36
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    One thing to check is whether there's a problem with your valve. For example, it may leak only when it's at a slight angle so when you reinflated your tyre you also straightened the value and it no longer leaks, or leaks so slowly you haven't noticed yet. Deflating over 24 hours indicates a slow puncture of some sort. Sometimes it leaks, sometimes it doesn't makes me think it's a valve problem.
    – Eric Nolan
    Aug 31 at 16:23
  • Can you rule out no one fiddled with your bike while unattended?
    – Jahaziel
    Sep 1 at 0:50
  • I've had a problem with a tube that has some sealant inside. Turns out the sealant is incompatible with the patches I had to use, preventing them from sicking to the tube, I could peel them off with little effort. Compounding the problem, Sometimes the mechanical pressure between the tube and tire sometimes helped the patch retain the air, sometimes not, so for this tire I sometimes had quick deflation (Mtb tire, 40 psi to 0 in one hour) and sometimes the tire remained at usable pressure for a week or so.
    – Jahaziel
    Sep 1 at 0:56
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Possible explanations:

  • Simple puncture that's somehow letting air out intermittently. Hard to imagine the physics of how that would work, and even it was happening somehow you need to replace or patch the tube as a practical matter.
  • Valve problem, as Eric Nolan points out in the comments. For example, the little seal inside a presta valve core may be torn or defective and shifting around as the valve is opened and closed.
  • In a tube with sealant, the sealant is depleted or otherwise becoming unable to seal a prior puncture. Any tube or tubeless tire where sealant is used will eventually reach this point if the sealant isn't replenished.

So, if you want to experiment/gamble, you could try replacing the valve core if it's a Schraeder or a removable core Presta tube. In doing this you might be able to simply see the torn seal, which gives you a pretty good guess that's what the problem was if so.

If there's sealant of any kind involved already, it's very likely you just need to add more.

Otherwise, replace the tube.

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  • Indeed, the Presta valve core thing just happened to me today. Tube went flat overnight -- took it out to check for punctures -- noticed the valve had come unseated. Unfortunately it can't just be popped back in because the moment you try to inflate, it ejects... Sep 1 at 3:12
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I had a puncture once that sounds similar to this.

I'd go for a ride and the next day my tire would be flat. I could fill up the tire and let the bike sit for days without any noticeable air loss. Then I'd go for another ride and within 6-12 hours, the tire would go flat again. I'd pull the tire and tube off and look for punctures and not find anything. I couldn't find anything in the tire casing and no holes in the tube, even if I inflated it and submerged it in water.

After several attempts at removing the tire and tube and searching for holes and punctures, I eventually found a tiny sliver of metal that had just barely punched through the tire casing. It was so small that it felt like a bump in the casing when I ran my finger across it. I couldn't get it out with my fingers or even tweezers. I had to get a knife and cut into the tube a bit to create enough space to get tweezers on it. Once I pulled it out and looked at it, it was no wider than a hair and maybe 1/4 inch long.

Once I found the sliver, I inflated the tube and submerged it in water again, but this time I overinflated it by quite a bit, so much that I was afraid it was going to burst from the pressure. Voila! Bubbles from a bunch of tiny holes.

I put in a new tube and was good to go.

I think what was happening was that the punctures were so small that they just didn't really leak without the metal sliver sitting inside of them to hold them open. And the sliver wouldn't create a new hole until I went for a ride and put some pressure and friction on it. But once it pushed through the tube, it held that new puncture open just enough for a slow leak.

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