I wouldn't count on teflon tape but a small O-ring should be a viable stop gap. If the O-ring is too thick or large, it will keep the collar from going on the main body far enough. The result will be that the battery tube will not contact the cathode ring and complete the ground path.
Josh,
Perhaps in loosening the components, you also loosened the bezel cap from seating the lens against the thin gasket. At any rate, you currently have a breach in the front end that needs to be fixed and is independent as you have stated from the other sealing area.
Just a side note here that probably is not a big issue but the blow test checks for positive pressure integrity. If you drop your light in water to any depth, negative pressure integrity is what you need to be concerned with. In most sealing designs, pressure differentials are maintained with a good seal regardless of positive or negative pressures. This is however not always the case. Many devices designed for diving will not seal properly in a negative pressure environment but their seal against positive pressure actually increases as the pressure increases. The gasket seal between the lens and reflector is just such a case where as the pressure against the lens increases, the compression on the gasket increases as well.
You could hold the lens and gasket with your finger against the reflector body and take the light underwater and with enough depth, the lens will stay in place due to the pressure and you will have a great seal until you return topside and have the lens fall off. Blowing bubbles with the lights does indicate a good seal if you can't get bubbles but it doesn't necessarily tell the whole story or in some cases insure proper function with submersion.
Sorry for the long aside here.
- Don