Was said: <Why does it develop a vacuum on cool down?>
Let's explore this. Let's convert this theory to facts.
Just use your favorite drink container. No, not a beer can.
1) You can use any container that has a small neck. Fill container with water, place a balloon over the bottle neck.
2) Put a pan of water on the stove, turn to high heat, place your drink container inside.
3) As it heats up, you will see the balloon expand. When it cools down, you should see balloon suck into bottle.
Heated coolant has excited molecules that get further apart from each other. As it cools, these molecules get closer together. It's a tremendous force.
I did all sorts of little experiments like that with my plane. Like I covered one exhaust pipe with rubber and watched how it behaved. I installed an atmospheric pressure sensor inside exhaust and watched the pressure pulses. There are a lot of physics principals that we don't have a good grip on because they are difficult to measure. I work in science museum where we expose people to these every day. It's a blast.
Al, although I find your tone a little condescending, I am sure that you don't mean it.
To use your example try the following experiment. Use the same bottle with the balloon over the top. But first put a little 50/50 coolant in the bottle. Heat the bottle, but you must boil a little of the coolant. Now cool the bottle. The balloon will never go all the way down. Not all of the vapor will go back into liquid form, there is a vapor pressure below which it will not drop.