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I'm not clear on why tire pressure decreases over time. I understand that air escapes through the tube. Possibly some leaks through the valve too? In any case, do tire / tube / valve combinations exist that will maintain a constant pressure over time?

Edit: From reading How long should a road tyre hold its pressure? I learn that air leaks through the tube material. I've changed the title in response - it originally applied to values only.

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A good quality valve, in good condition, will leak hardly any air at all. Tires loose pressure mainly because air diffuses through the inner tube wall. Some fancy-dancy racing tubes are quite bad in this regard. – Daniel R Hicks Jul 4 '14 at 22:17
Daniel R Hicks and Blam responded to my original question which referred to valves only. – Paul Carey Jul 4 '14 at 22:20
As I indicated, there are some tubes (lightweight racing tubes) that are worse than normal with regard to holding air. But I've never seen a tube advertised as holding air longer. I'm sure it could be done, but no one's developed a market for such a tube. – Daniel R Hicks Jul 4 '14 at 22:25
The problem is that diffusion rate is proportional to wall thickness, material and external conditions being the same. But thicker walls make for heavier tubes and cyclists don't like that. I suspect manufacturers optimise wall thickness quite enthusiastically. – Móż Jul 4 '14 at 22:28
You could fill the tubes with a gas that diffuses more slowly - the "advantages of nitrogen" question is helpful there. – Móż Jul 4 '14 at 22:30

You assume the whole leak is the valve. Rubber is not air tight. Most of the leak is through the rubber.

Clearly can create a near airtight valve. And with enough rubber a near air tight tube. But it would be heavy. Just pump your tires once a week. I see the link said pump daily. OK, at least once a week. On my road bike I pump before every ride.

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