Quote Originally Posted by csr1210 View Post
I had the screw cap part break off of one of my valves (definitely an "oh sh!t moment), and I was able to screw it back on. (the post itself was intact) I don't know if I just got really lucky, but it still works. I do have to be careful to gently unscrew the valve now, because it has come off a couple more times, but each time (after finding the @#!@ tiny little piece on the floor) was able to replace it.
I think you're talking about the removable core that Schrader tubes (and a few Presta) have. There is actually a little tool which can be used to get the Schrader core screwed back in a little more deeply. If you have Presta tubes, just use a very fine hemostat or tweezers instead. Apply a very, very tiny amount of blue LocTite to the threads first.