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I noticed that my back tyre was losing pressure -- on the order of 70 psi down to under 30 psi in three days. (I pumped it up and left the bike alone for those three days.) I thought it had a small puncture, so I took the tube off and examined it very carefully, dipping it in water to find any leaks. I did not find any.

After remounting the tube and filling it up, it is currently (after four days) holding the pressure well.

Is this something to be worried about? Temporary valve malfunction or something else?

EDIT: Now, one and a half weeks since the last pumping-up, the tube had indeed again lost much of its pressure, though the pressure loss was quite a bit slower this time. I am betting the cause is a badly applied patch. (How can I find which one it is?) Should consider just replacing the tube.

  • Similar, although a different valve (mine's a Schrader): bicycles.stackexchange.com/questions/24407 – oals Aug 12 '16 at 11:12
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    Could have been dust in the valve. – andy256 Aug 12 '16 at 11:20
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    Was there a large fluctuation in temperature between the first filling and then storage? And then you re-pumped it up at the same temp it had now been stored at? – Nate W Aug 12 '16 at 20:20
  • In addition to the valve reseating itself, it's not impossible that there was a pinhole, and some piece of debris simply sealed it from the inside. Bigger apertures than a pinhole leak get clogged: kitchen sinks, toilets. – Kaz Aug 12 '16 at 23:01
  • @NateWengert Nope, stable temperatures all the way. Not thermal expansion. – oals Aug 13 '16 at 7:19
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In the end, it seems I had a badly applied patch that was leaking.

I dipped the tube in water (again) and there were small air bubbles coming from the edge of the patch, maybe one per second. I tore off the patch and installed a new one.

It leaked more when I rode the bike, likely due to the extra weight. When I wasn't riding it, the air loss was much slower which lead me to think that it had fixed itself. Also, my pressure checks were inaccurate, done with fingers only, which can't really tell the difference between 40 PSI (the minimum) and 75.

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If the tube is not leaking its likely the valve, if its a shrader type sometimes they dont reseat propery as the inner pin goes slightly off center, or the valves rubber seal does not seat properly. With shrader valves you can replace the core with a valve tool If its a presta valve, depending on type only some can be replaced.

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