At least it was a "slow" leak, keeping rideable pressure for about 1/4 mile, so I only had to stop 8 or 9 times on the way to work to re-air it up.
I began to feel it just as I was heading out of my side street to 29th Drive, sort of a wallowing, and I had no time to try to stop and fix the leak, only to keep adding air when it got unrideable (around 10-15PSI).
Same thing on the way home from work. Although this part wasnt' really time-critical, I didn't feel like sitting in the dark taking the bike apart to deal with it, so I just rode home with frequent stops.
It was frustrating, because not only do I have the slime tire liner, I also have a cut-open-on-it's-inner-circumference nice thick tube as a secondary liner including for the sidewalls, and then chunky slime in the tube itself. So, basically there should be very little that gets thru in the first place, and if it does get thru it ought to be plugged up quick.
But this didnt' fail in any place that could be helped by any of that--it was in the valve stem, AGAIN:
I carefully examined the rim's valve hole and found no irregularities or sharp edges, and the crack in the stem doesn't line up with either the inner or outer rim layers anyway. So I guess it's just one more in the long line of valve stem failures from poor manufacturing that I have had over the last several years.
I am SICK and TIRED (haha) of flats.
So I decided to make myself a Twinnertube, as others have done before:
I drilled a second valve hole in the rim, 180 degrees from the original. Then I went thru my four remaining mostly-usable innertubes, and found one with no patches yet, and one with only one, and both held air fine the whole time I had them stored in the wheel closet, since the previous flat problem a couple weeks ago, I think it was. There were actually seven tubes put away then, but one of those also had a failed valve stem, and two others simply had so many little holes that I don't have any way to patch them all.
So I took two of the four remainign ones, and installed them in the tire/slimeliner/tubeliner previously on the bike, and then onto the rim. I'll tel you--if you think getting a single tube and tight bead tire onto a rim is tough, doing it with what amounts to three tubes (the tubeliners and two actual tubes) in there is danged near impossible. Took perhaps 45 minutes to get the tire on there without damaging either tube.
Then I aired each up a little, 20-30PSI, let it all out, reaired, let it out, etc., to help them seat against each other. When I could feel/hear no more shuffling or "creaking" inside, as the tubes moved during air-up, I filled each about 30PSI for a total of 60PSI, and mounted it back on the bike.
This means that assuming there isn't any rubbing between them or the tubeliner or rim, and nothing wears thru, then when eventually one of these two fails suddenly and catastrophically, the other tube still has pressure in it, so I don't lose control, etc. Even if it's just a regular failure, slow leak, etc., I still have the other tube to keep me going, and if I need to I can just air that tube up to full pressure.
This is just theory, though, siince I've never tried this before. I hope I never get to test it.
While putting the wheel back on teh bike, I noticed that the rim is pretty deeply dented at one point. Basically a big flat spot. I can't true it out, either, so I'll just have to live with it.