Presta Tubes - Losing Air - Culprit Found!
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.