Can Presta valves suddenly lose pressure? 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?
Possibly the valve was contaminated with a particle or dirt and was not sealing properly leading to quick but not instantaneous loss of pressure. When re-inflated the contamination was dislodged and the tube holds pressure once again.