Twice in the last week my inner tubes have leaked through the valve. In the latest one, it was a brand-new tube that I took out and within a couple of hours punctured on the road. At home, I fixed the leak, tested the tube, then deflated it and replaced it inside the tyre. On refitting it to the bike, I began inflating it but after a few seconds it lost all its air through the valve. Why would this be? How can I stop it happening? I'm buying too many new tubes.