Quote Originally Posted by zia View Post
My valves actually look a little crooked and bent - maybe that's part of the problem.
If the valve core isn't perfectly lined up with the stem after you unscrew it, that's normal. There's no spring in there as there is with Schraeder valves, so it will just sort of rattle around when the valve is open, especially when there's not much air in the tube. That's what makes it hard to start a fresh tube. (Hint on mouth inflation: bite the barrel between your front teeth to hold the valve open. But, mouth inflation to a third full? That woman must have some monster sealing lips... I'm not even going there. )

If the valve core is crooked when it's screwed down in the closed position, then it should be replaced. (The cores are replaceable, but may be hard to find, so you'll probably wind up replacing the whole tube.)

If the whole stem is bent, then definitely replace the tube.

If the whole stem is coming out of the rim to where it's not parallel with the adjacent spokes, then you need to take the tire off and re-align the tube. Misalignment of the stem stresses the tube where it attaches the valve stem, and can eventually cut the tube.

HTH...