The P72 and the P92 chassis have pressure-type engine cooling systems with thermostatic control of the coolant circulation. The cooling system is sealed by a pressure-type radiator cap that causes the system to operate at higher than atmospheric pressure. The high pressure operation raises the boiling point of the coolant, thereby increasing the efficiency of the radiator. The 110 kPa (16 psi) raises the boiling point of the coolant to about 125 ° C (257 ° F) at sea level.

The pressure/vacuum valve radiator cap allows the coolant to expand through the pressure valve in the center of the cap without building unnecessary pressure. The expanding coolant flows through the pressure valve in the center of the cap into the surge tank reservoir. The vent valve closes due to expansion and coolant flow. The nominal 110 kPa (16 psi) pressure will not be reached until the system is working at maximum capacity. Any air or vapor in the cooling system will be forced to the radiator surge tank and out through the vent tube at the top of the reservoir. As the system cools, the extra coolant in the surge tank reservoir will be drawn back to the radiator through the vent valve. In this manner, the radiator will keep itself full at all times.