I Finally Figured it Out!
That intermittent leak in my rear tire was driving me nuts!
Sometimes it would stay full for days, this morning after topping it off it was down in 2 miles.
In the process of trying to discover the cause, I have:
1. Removed the tube and turned the tire inside out, checking for anythning, feeling inside, using a kleenex, etc.
2. Placed the tube, as full as I could get it, in water - no leak.
3. Placed the pressurized tire on the wheel in a water trough, and turning it slowly, including the valve covered by water - no leak.
I figured it might be an intermittent high pressure leak in the Goop-filled tube, sealing, leaking and resealing. If so, I didn't want to put another tube in without knowing the cause of the leak in the tire.
This morning, after going out 2 miles and then returning with a very low tire, I messed around a bit with the presta valve. I finally discovered that it was intermittently not seating properly, and I had not been rigorous about really tightening the valve stem screw.
So, I pumped her up one more time, really tightened down the presta valve, and, lo and behold, it held just fine.
As this is a pretty expensive tube, and I think I know the cause, I will not replace the tube at this time. If it continues to lose air, even after tightening the valve, I will.
At least it is not some devious thorn in the tire!
Anyone else have strange, unexplainable flats?