For the sake of clarity, and without any goal of bursting balls of anybody, the argument made in the first post is wrong IMHO.
Pumping with all the tube filled with water, it is less effective than pumping with air.
As I understand it, 5 hg is the force that is pulling whatever is in the tube. So lets say 5hg = 1 lbs (or 2 or whatever) for each squared centimeter. If something in the tube is less expandable than your penis, what you have is that your penis is pulled but can’t expand. Think as if the cylinder is filled with concrete, your penis inside: 1lbs is pulling the concrete, 1 lbs is pulling your penis. But the concrete is not expanded, your penis neither.
What is tricking here is the feeling: people feel more force because they feel both forces: the pulling force of the vacuum, the pushing force of the water.
Pumping with a tube partly filled with water can be more effective for girth, for the same reason: it would be like pumping in a narrower tube.