Originally Posted by
fizztpok
The valve wasn't broken on the replacement tube like I thought. I just got a little overzealous with the hand pump.
It's a little tough to see, but the rubber separated where the stem connects.
I had that problem on an old wheel; it was caused in part by a slightly sharp edge on the inside of the valve hole in the rim. I replaced the rim tape, and made sure the tape covered the hole really well (I didn't use the hole already in the tape, I just covered the hole with tape and then bodged a hole through it, meaning the excess tape kind of covered the edge of the hole as I pushed the valve through.)
Just a thought.