"How do you squeeze a soda bottle over and over again?"
I used 1 liter soda bottle. Squeezed out about 1/4 liter, then unscrewed Squeeze enough to let air in, repeat...
Now maybe a soda bottle is designed to hold pressure in an expanded shape, and not be crinkled down. That might cause it to fail eventually. Mine is a bit wrinkled after 4 days of use. Might have to replace it occasionally.
The main reason for using this, is to make it easier to carry 3.5 liters of water in my pack. It doesn't seem like water bag like Platypus, Sawyer, or Evernew would be strong enough to hold water in my pack?