CamelBak Big Bite Valve
CamelBak's patented Big Bite Valve mouthpiece offers hands-free hydration and is ideal for any high-performance situation. Easy to use just bite and sip! Self-seals after each drink to eliminate annoying drips.
5 stars to CamelBak for even selling them. I've used these to convert a no-name hydration pack as well as replace my CamelBak valve after a couple years of use. Mine began to drip after ~2 years of consistent use (2-3x a week), which I think is acceptable for a wear item like this.
The only complaint that I have is that it is difficult to to remove from the tube, and I think that could break it, so you have to be very carefull...