I could not figure out why I was losing air in my presta tubes, then I figured it out... the valve cores had come a little loose
- when submerged completely underwater, I could see air bubbles coming from the valves (and nowhere else on multiple tubes that had the same symptoms). At first I had not submerged the valves themselves, thinking the problem was a pinprick puncture in the tube body.
The solution, just tighten the cores - there are flat faces/sides on the cores that let you use a suitable wrench or set of pliers to twist them.
Figured I might post this for anyone else with the same maddening symptoms.
These were Schwalbe 6 and 6A tubes.
Thanks for the post. I've had some problems with Schwalbe tubes in the past and stumbled across some Continental tubes that I run in some of my folders. I've never so much as had a flat on those tubes.
I had three tubes in a row go recently with no visible problems whatsoever. I dropped in my Continentals without doing any other thing and no problems at all. At this point I'm convinced it was a quality control problem and not something making the tubes go flat.