I had a Flexi Flask, with the straw at the top. I found that the straw would come loose and float around inside the bladder. Also, pressure inside the pack would force water out of the top after the cap came loose from the tube/cap not being able to move freely with movement. You would have to be sure any pull on the tube was in the direction to make it tighter not looser. Sounded good at the time with the cap at the top of the bladder. I have found that my Platy has a better designed cap with a "o" ring in the cap and designed to let the tube move a little withour unscrewing the cap. Worked great this summer on my trip.

Are you sure that isn't a Everclear bladder?:)