Abstract: A hydration system for backpackers or other athletes including a bladder, a filling opening, an enclosing cover and filling opening, a flexible line and a deformable valve to be held in the user's mouth. The cover is of smaller volume than the bladder to protect the bladder from bursting upon overfilling. The valve is operated by pressure in the mouth, usually by the user's jaw and teeth to open a slit and allow potable liquid to flow from the bladder through the line and into the flexible valve for the user's consumption. The slit is located in the side wall of a membrane having a reverse fold whereby pressure from the bladder, in the absence of user applied valve deforming pressure tends to close rather than open the slit. The line includes a quick disconnect fitting to allow the filing of the bladder from the line as well as from its own filler opening.