Originally Posted by
billyymc
One other t hing to check is the valve stem core -- could be loose. I've had schrader valves where the stem core is just a little loose and they slowly lose air. Not sure if that can occur with prestas - I've never had it happen.
Was just going to suggest this... I had a presta fail about a month ago like this. The failure however was a bit more obvious, I just had not been in a quiet place to notice the low hiss coming from the valve. I have no idea what exactly failed in the valve; but I know when it was under pressure, in the tire, it slowly leaked. But out of the tire, when it wasn't "compressed" it showed no signs of leaking (even passed the water tank test).
I didn't troubleshoot it any more, just slapped a new one in and went on.