S/T truck models use a pressure type engine cooling system. The coolant circulation is thermostatically controlled. This provides for a fast initial warm-up. Then this system keeps the engine operating temperature in the normal range. This is regardless of load or outside temperatures. The radiator is sealed by a cap which allows the system to operate at 150 kPa (15 psi) higher, than atmospheric pressure. This raises the boiling point of the coolant to about 125°C (257°F), at sea level. This results in increased cooling system efficiency. The normal operating pressure is not reached until the system is working at maximum capacity. Heat from engine operation causes the coolant to expand. A pressure-vacuum valve in the radiator cap prevents the build-up of unnecessary pressure. The pressure valve opens to allow expanding coolant to flow from the radiator into the coolant recovery reservoir. As the system cools, a vacuum is created by the contracting coolant. The vacuum valve opens, drawing coolant from the coolant recovery reservoir back into the radiator. This system keeps the radiator full at all times. The cooling system also directs some coolant through hoses to the coolant heater core. This system provides windshield defrost and passenger compartment heating.