I've search a few threads and seen the above statement, but I am not sure how it works. How does this happen when the spring loaded valve on top of the AST appears to check the flow in the reverse (sucked) direction. It seems you reach 9 psi and the coolant would leave the AST to the overflow tank but then spring loaded cap would not let the coolant back in from the overflow tank since it is seated on the AST lip. Am I missing something simple? I can understand the cooland going back and forth between the engine and AST.

 

The reason theres a over flow tank is because coolant expands when heated, so when you turn your car on and the coolant heats up it needs to go somewhere and you dont want it to rupture your hoses or bleed out the clamps so theres the check valve on your radiator cap that lets the coolant escape to your overflow tank. Then when your coolant cools it shrinks. Theres actually two check valves in your radiator cap the seconds a little harder to see that one lets coolant come back in when theres enough vacuum to pull the coolant back in.