I was doing my usual 15 mile round, about mile or two to go, when suddenly I realised my front tyre had gone flat. It has heavy duty, mountain bike style, valves. I haven't been able to find a puncture, re-inflated the tyre and so far it's holding 40psi. I rode it for a couple of miles without any problems. Could it be that the knurled collar came loose?
Some (not obvious) problems with tires lead to very subtle punctures in the tubes. These punctures can only be found when the tube is inflated outside the tire and then checked either under water or by trying to feel the air going out by hand. They are not seen on deflated tube. When mounted, such a tube/tire holds the pressure but would spontaneously deflate at some point within a week or about.
Once I was able to locate the puncture - a very thin 4 mm cut in the outer side. It was a very easy tire/rim combination with all mounting done with plastic levers, so I do not believe I damaged the tube or tire myself, and this tire deflated multiple new tubes before I was forced to replace it.
Hence the valve may not be the problem, and if you screw it closed (you only need to screw it open to put the air in) and put the plastic cap properly it is unlikely to be seriously contaminated.