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I've been having a problem with a "flat" tire - I cannot get the air pressure over 5-10 psi (35–70 kPa), I can hear air whistling out if I pump more in. Disassembled, the tube inflates up to any pressure I want, and I can leave it for days without it losing air. Reinstalling the tube, it behaves flat again and refuses to inflate - this is instant with auditory and visual cues, not a slow leak. Back out, the tube acts unpunctured and will easily go up to 50 psi (350 kPa).

I can't find anything inside the tire or rim that might be puncturing it. I'm aware that replacing the inner tube is an option, but I can't see a reason to do that if the tube appears undamaged, and I don't know what to do if that happens with the next tube.

Tire is a Rene Herse, rim is a Stan's, tube is a Kenda.

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    What kind of valve do you have?
    – Willeke ♦
    Commented Jun 18 at 20:54
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    It's a presta valve.
    – Jon Bon
    Commented Jun 18 at 20:55
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    With the tube out and inflated to "firm", try flexing it all around and gently pulling the tube. Remember your mouth and lips are most sensitive to airflow, more so than hands. Pass the tube in front of your mouth to search out the leak. Be suspicious of previous patches and the valve too.
    – Criggie ♦
    Commented Jun 18 at 23:28
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    @Criggie I thought I'm the only weirdo who searches for leaks with his lips! ;-) That method is especially helpful in noisy environments (like on the side of a road!). It helps a lot to wet the lips; the cooling effect from evaporation is a clear sensation as well.
    – Peter - Reinstate Monica
    Commented Jun 19 at 7:56
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    OTOH (@ Peter and Criggie) that method doesn't work so well if you're out in the wind
    – Chris H
    Commented Jun 19 at 8:17

2 Answers 2

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When you inflate a tube that is not inside a tire, even if it balloons up a huge amount, you are only getting maybe 1-2 PSI (7–14 kPa). A tube that is not constrained by a tire will increase in size without increasing the pressure inside the tube much at all. So you apparently have a very small puncture that doesn't open up until 5-10 PSI (35–70 kPa) that you can attain while the tube is installed in a tire.

Try inflating the bare tube as much as you dare to and dunk sections of it into a bucket of water and look for the smallest of bubbles on the surface of the tube. That's often the only way you'll find a small puncture like that to be able to patch it.

It could be a puncture in the tube, or as others have hinted at it could be a problem with the valve, so include that in your dunking exercise.

And sometimes you never can find that tiny leak, which means you just discard the tube and use a new one.

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    And then don't forget to look for the little thorn or glass shard embedded in the outer tire. For that, make sure not to rotate the tire relative to the rim; if you do or even take it off, mark the valve position on the tire first. For little holes, the embedded objects can be quite hard to find because they don't poke through much. Inspect visually and tactilely (yes, that word exists ;-) ). Work the tire between your fingers and thumbs both for detection and removal. Last time I needed a needle to get the shard out.
    – Peter - Reinstate Monica
    Commented Jun 19 at 8:00
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    For an indication of pressure in a tube without a tyre, see this answer of mine where I test it. And when dunking, you might need to wipe off bubbles clinging to the tube before checking if new ones form (soapy water can help but I prefer to keep soap off my tubes).
    – Chris H
    Commented Jun 19 at 8:21
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    @Peter-ReinstateMonica a habit of lining up the logo on the tyre with the valve stem helps here; even if you flip the tube there are only 2 areas to inspect. I've had punctures at the roadside where finding the thing in the tyre by feel was the only way to track down the hole. But of course there's risk of cutting yourself doing it by feel. So I inspect inside, then outside (one of the many little cuts is likely to be the cause, especially with puncture-protected tyres), then feel inside. Some thorns, bits or wire etc are better pushed/pulled inwards rather than outwards, small pliers help
    – Chris H
    Commented Jun 19 at 8:25
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    @suma, I find it hard to believe that a tube can be inflated to 50 PSI outside the constraints of a tire without bursting.
    – Ken Carlson
    Commented Jun 19 at 12:21
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    @Suma Ken's right. My test showed a split forming under 10 PSI, when the tube was several times its designed size. If it could somehow get to 50psi without bursting it would be hugely stretched and have to be stuffed into the tyre afterwards
    – Chris H
    Commented Jun 20 at 8:17
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When searching for a tiny air leak, water is your friend.

Your leak is tiny, at least at pressures available in a bare tube.

Get a container full of water where you can immerse at least a section of the tube, immerse it and the bubbles will reveal the truth. Streching, bending, etc are not forbidden either.

Just remember to completely dry the tube before any repair attempts.

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