Originally Posted by turbodrx7
That is the EXACT same thing that i said...
No it isn't. this is what you said: "when the system presurizes ... So it pushes some coolant into your overflow. When you turn the car off the pressure that forced the coolant out acts as a vaccum to pull the coolant back from the overflow..."
In other words, "the pressure that forced the coolant acts as a vaccum ..."
Not correct. Why?
Take a cold engine, pressurize it with a coolant pressure tester (Stant product) until coolant starts overflowing. Leave the whole thing alone overnight and if your car coolant system is in good shape, there should be no pressure drop nor coolant flowing back into the engine.
Close but not quite the same.