So, yesterday, for an Australia-wide initiative originating in Victoria called 'Ride to Work Day', I do the same.
Pretty much halfway in, again, this...
I noticed the back was going a little squirmy, and look down to see a slowly deflating rear. Stop, climb off, break out the mini-pump. The second I put the pump head on and flicked the lever, the valve completely detaches itself from the tube.
So, quick change, fresh tube, pump it to 60PSI (about as good as I can get with the mini-pump and enough to get me riding), and off to work.
I was trying to figure out why this happened, as its never happened to me before. I'm thinking maybe I ran the ring too tight against the rim, and in using the floor pump to hit 110PSI its stressed the joint so much that riding has pushed the limits of the tube. That, plus a 90kg rider, and a few bumps on the way has seen the flex in the valve overcome the tube. Cause the valve was pretty tight against the rim it has had nowhere to go. Except for out.
Maybe if I'm doing this more often I should get some 28C wheels instead of 23C's.
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