What would cause a vacuum in the cooling system? After my car has cooled the top radiator hose is collapsed. When I open the radiator cap I can hear air rush in. Isn’t the cooling system supposed to draw coolant from the reservoir as the car cools off and the volume of the coolant shrinks due to the cooling? There is coolant in the reservoir. I’m certain I’m going to have to replace that top hose. That hose now looks deformed from being collapsed for an extended period of time.