radiator and over flow question

[Stocker]

Originally Posted by purple gas

so, does that mean when the prerasure valve in the cap closes, and the "coolant" cools down there is a negative preasure, or in essence a vacume within the system?

Yup - so depending on how you visualize it, either the vacuum within the system sucks the coolant out of the overflow tank, or - when atmospheric pressure exceeds the pressure within the system, it (atmospheric pressure) pushes the coolant back into the radiator. Same difference.