A radiator cap for a vehicle cooling system of a basically closed type includes a first valve which is open when the interior of the radiator is at atmospheric pressure, and remains open until fluid flow out of the radiator closes it. A second, or overpressure, valve in the radiator cap functions to vent the radiator only if potentially damaging excessive pressure exists within the radiator. The normal operating pressure range of the radiator is between the closing pressure of the first valve and the opening pressure of the second valve. When a low vacuum exists within the radiator, the first valve opens, permitting a metered flow of liquid coolant from a coolant recovery bottle back into the cooling system radiator. When a high vacuum is present within the radiator, the first valve opens further to permit higher flow rates of coolant back into the radiator.