It's basic suckage & drawage - lessee if I can further confuse the issue. It doesn't blow out the overflow until the pressure rating of the cap is exceeded. With a sealed system, the radiator is always full, so with expansion when heated, some coolant will be forced out, and caught in the tank. As it cools it contracts, and if there is a sealed-system type cap in place, coolant will be sucked back into the radiator through the hose that ends
near the bottom of the tank. Make any sense now?
