Like I said, we humans refer to it as sucking because that's how we understand it from a young age.
however engineers and physicist know that it is actually air pressure pushing on the liquid lifting it up because something ( in this case the pump) is creating a lower pressure location at a higher elevation than the liquid level open to atmosphere.
If the air pressure over the liquid inside say a closed vessel was actually 0.5 barg, 1.5bara, this would allow water to flow out of the top of a pipe 5m higher than liquid level (just). At that point you wouldn't say the air was "sucking" it out of the vessel, you would describe it as being pushed out by the pressure above the air.
Now subtract 0.5 bar from both sides - pressure above the liquid 0 barg, 1.0 bara, pressure at top of the tube -0.5 barg, 0.5 bara - now it suddenly become "sucking"??? No. It's the same process.
All self priming pumps need to be partly filled with liquid to work.
To clear a line of air which has vertical down flow, you need a liquid velocity of >1 m/sec. A bit less might do it for different fluids, but 1m/sec is just about guaranteed. Anything less and the air stays trapped in the highest point.
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