# The effect of pressure gradient across dissimilar fluids

So my question is - would a strong enough negative pressure be able to pull a gas through a liquid? I can draw a diagram if anyone needs it but I'm trying to figure out what would happen in the following situation. Imagine you had a solid pipe that formed a large U shape with one end sealed, you fill the pipe halfway with water so that the bottom of both legs of the U are filled with water and the tops contain air. I understand that creating a vacuum at the open end would cause the water level on the closed end to drop, but would it have to drop to the point where the water level goes below the bottom of the U for the air to bubble out and be replaced by water rapidly or would the air start slowly flowing through the water at a slow rate as soon as the pressure gradient is created?

The real application of this I'm trying to understand would be similar to how they use vacuum systems in injection molding. Imagine you were trying to make something with a mold that looked like a solid steel sombrero in a box. If you filled the base of the mold with your fluid, created a low pressure area above the fluid, and the fluid had a pathway into the high center part of the mold, with enough time would the recessed center area eventually fill with the fluid? Obviously in real life you'd turn the mold upside down and fill it, but I'm just trying to conceptualize what would happen in this situation.