"Leaks from bite valve" - Review of CamelBak 1L Bottle w/ Big Bite Valve

I commute every day to the city and gain or lose about a thousand feet. The bite valve, while a great idea, does not seal well and I end up having to carry the bottle in a plastic bag to keep my other stuff dry. No one else seems to have this problem. Is it just the altitude, or is there something else going on? My Nalgene does not leak but, of course, does not have the bite valve. Thought it might have been a defective bottle but the next one had the same leak. I have three bottles now, and they all leak.
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