I won't encounter difficult with the bag in the upright position because the positive pressure & gravity get the water flowing to the valve. I will in-counter problems with the pack on it's side or laying down because the hose is no longer at the bottom of the bag and GRAVITY is no longer a factor in the pressure system and can't push the water to the bite valve, you have to SUCK all the air out and water to get some.
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Rule #10 // It never gets easier, you just go faster.