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My back tire got flat. If I pump it up now, it can hold the air for about 30 minutes to an hour before it's totally flat.

I took out the inner-tube yesterday, pump in a bit air and put it into water, trying to find a hole but couldn't find anything.

Could it be that the tire is too old and I must replace it?

EDIT: thanks to all your response. Beside soap water, I found out hot water is really helpful and that was what I used to fix my tire. There was a very tiny hole on the tube that I couldn't find when I used room temperature water.

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Added the puncture tag. –  Neil Fein Jun 12 '12 at 15:57
    
Once you find the puncture, do not forget to inspect the tire in search for the offending object. Inspect the tire casing very carefully, feeling with your fingers and in a well lit area to help with visual inspection. Inspect the inside and the outside. Many times, very small holes are caused by objects that became embeded in the tire but did not pass all the way trough, so they may be pushed further inside the tire wile riding, causing a new puncture very close to te old one. –  Jahaziel Aug 9 '13 at 0:34
    
This makes me feel wasteful—I would have just swapped for a new inner tube and trashed the flaky one. –  Drew Stephens Aug 9 '13 at 12:00
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4 Answers 4

up vote 11 down vote accepted

Sometimes it can be hard to find a small hole. Sometimes it helps to mix water and dish soap together and either pour that over the partially inflated tire, scanning for leaks or submerging the tire in a bucket of soapy water. The soap will bubble and make holes more apparent. You also probably want to immerse the valve to see if the leak may be originating from there.

If the problem is coming from the valve, then you will want to go ahead and just replace the tube.

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Thanks, Benzo. I'll try it this evening. –  chepukha Jun 12 '12 at 15:39
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Take the tube out and pump it up so that it balloons to 2-3 times its normal size. There have been times when the puncture is too small to detect in normal situations, even soapy water, but this has never failed me! Even the smallest puncture, like from a brake cable sliver, will show itself. Listen for the hiss and then mark it with a pen to find it after you deflate it (Usually I make 4 lines coming out away from the hole, like an X but with the middle taken out, this pic is just an X, but the 4 lines makes it easier to pinpoint).

If this doesn't produce any results it might be in the valve. If you have a Schrader, get your finger wet and put it lightly over the top (I spit on the ground and put my finger in it) this will allow you to feel any air leaking out. With a Presta it is a little harder, but wrap your wet thumb and index finder around the top of the valve to check it. Most of the time you can fix the Schrader using a core tool, not sure you can do anything to fix a Presta (but aside from a bent core bolt I've never seen a Presta valve leak) enter image description here

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Thanks for your effort and detail photo. I usually find it difficult to take the back wheel off. So I only take the innertube out of the out of the tire; hence, I can't pump the tube that big. –  chepukha Jun 14 '12 at 3:19
    
This is overly complicated. I cannot imagine ever laying bike tubes into a bed of cat litter for any reason. –  user313 Jun 14 '12 at 6:39
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@user313 cat litter?? What answer did you read? –  Brad Aug 9 '13 at 21:55
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To answer the question directly, yes, if your tube is losing air that quickly, it needs repair. It is not a matter of simply being too old.

There is likely a very small hole or a leak in the valve.

Replace the tube, or patch it. Benzo's soapy water suggestion will work very well.

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My favorite method is to remove the tube from the tire, reinflate it a bit, and completely submerse it in a swimming pool. I've used this to quickly identify leaks that were so slow they took days to deflate the tube.

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