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I've already seen it happen at least 4 times in my back tyre and once in the front one.

After installation of the new tube, after some time of cycling (days) the tube will start deflating. Most of the cases it was very slow so that I needed to pump it every 4-5 days. The latest is much faster.

I checked some of the tubes and couldn't spot any hole, even when submerging in water.

I have always cleaned and checked the tyre itself and couldn't find any debris that could be re-puncturing the tubes. Same issue with winter tyres. I have used both for a total of 7000km, they look in shape tho, the patterns are still very visible.

Still using the same brand/type of inner tube, they worked fine until a year ago when this issue had started.

What could it be?

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    Most likely would seem something on the rim re-puncturing the tubes, but then you should be able to see those punctures when submerging the inflated tube. Perhaps the problem is with the valves rather than the rubber? What types of valve are those? Commented May 5 at 14:10
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    But even then, wouldn’t the valve(s) show air bubbles as well? Maybe in looking for a leak in the tube, possibly overlooked the valve too quickly, passing it by and not observing it when immersing the tube in water.
    – Ted Hohl
    Commented May 5 at 14:54
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    When looking for a slow leak, one has to go slow when observing for air bubbles. Sometimes they are obvious, but a slow leak can require patience to identify the flaw.
    – Ted Hohl
    Commented May 5 at 14:57
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    Patience and sometimes some light manipulation. Most people use a small container of water, which means a tear might end up at a bend which can conceal the issue. If your ears are good, they are also quite good at localizing a leak
    – WornChain
    Commented May 5 at 16:13

2 Answers 2

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Most of the cases it was very slow so that I needed to pump it every 4-5 days.

vs

couldn't spot any hole, even when submerging in water.

If the hole is so tiny, said otherwise if the leak is so small, you have some chance only inflating the tube with some relevant pressure (but try to avoid making the leak noticeable with an explosion ;) ) and then checking it submerged. You still need to squeeze it, though, and you need a large bucket.

An alternative is to put some grease on the valve, if those are tubes with detachable valve. Sometimes air finds a way out of them through the valve (no, the screw + cap assembly is not air tight).

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Is it a presta valve? You might need to tighten the inner valve core.

See this video and let me know if it has improved

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