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I ride a Btwin MyBike 7s.

Yesterday morning I noticed (for the first time since purchase) that in my rear tire, the tire pressure has decreased to a flat. I thought it was a puncture, so I refilled the tire using a hand-pump and poured water over the tires expecting bubbles but didn't get any.

Then I took it for a ride, 20 kms and came back - nothing happened.

Then I came back in the evening to notice that the air pressure has decreased again, almost half. Then today morning it's a flat again.

Is it a puncture? I checked the tire's mouth valve, it's not leaking any air there. Should I get the tube out and check for a puncture?

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  • It's normal for a tire to lose maybe 2-5 PSI per day, but you're case indicates a slow leak. Unless it's a Shrader valve and the core is simply not screwed in tight you will need to replace the tube.
    – Daniel R Hicks
    Dec 9 '15 at 13:05
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Answer: Your tube has a slow leak, this cannot be found by pouring water on the outside of the tyre.

Get the tube out of the tyre, which means unmounting the wheel and maybe releasing the brakes.

Inflate the bare tube to about 5 PSI, no more than a couple of strokes of the pump.

Slowly submerge the tube, one section at a time into a bowl of clean plain water. Look for bubbles to form and rise to the surface.

Since the tube takes hours to go flat, it won't be a big hole Once you find it, mark with some tape or chalk or something.

Dry the tube, and run through the patching process. Remember to buff the tube lightly, apply enough fluid to cover an area bigger than your patch, leave the fluid 5-10 minutes to dry completely before applying the patch, and to roll the patch on hard using a round coin or other pressure - fingers aren't enough.

Then you can inflate the tube to 5 psi and check for more leaks... there can easily be multiple holes.

If your valve is leaking, just buy another tube of the same dimensions and fit it carefully.

If you've never done this kind of thing before, ask family and friends for some hands-on help. Its not hard to do, but some things are better watched than explained.

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    The key is in the first paragraph (+1).you can't find a puncture by pouring water on a (tubed) tyre. But I suggest putting a new tube in and leaving the patched one to set nicely then at 10psi for a few days to check it (as it's the first time patching). Don't forget to check the tyre inside and out for what could have caused the puncture, and inspect the rim tape.
    – Chris H
    Dec 10 '15 at 7:02
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    If you are having trouble seeing bubbles, sometimes adding a very small amount of dish soap helps in identifying leaks. You should take care, being careful not to agitate it too much creating extra bubbles which may make it harder to find the puncture.
    – Benzo
    Dec 11 '15 at 23:36
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It could be a really small puncture or it could be a damaged valve. I recommend you to replace the tube. They are not that expensive and you have to get the tube out anyway.

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