There are several questions that got my mind pondering.

I know in the old systems there is a hose that goes from under the radiator cap to the overflow tube. That usually just dumps on the ground when the car gets hot and pukes out coolant. Then when it cools down, the coolant level drops leaving an air gap at the top of the radiator.

Does the radiator crate a vacuum and suck air in when cooling down, or does the coolant just shrink (not sure of the correct term, but it expands when hot) and the level settles where it needs to be?

If you have an overflow tank that has a hose entering from the bottom, when the system cools back down, it will draw coolant back into the system and no air. What about one that the tube enters from the top and goes to the bottom of the overflow tank? Wouldn't there be a small air gap where the tube enters the overflow tank and the top of the coolant in the overflow? Then when the vacuum is created the small air pocket would get sucked in as well as fluid leaving the radiator a tad lower than full, but more full than without a tank, correct? There should be either a vented cap or another hose going to the ground to keep the tank from collapsing on itself is what I am thinking.

If you were to blow into the overflow hose when the engine is cold (simulating vacuum draw), would you get any leaks or hissing sounds from the radiator cap area? What about if you blew in from the hose going to the ground from the overflow tank?