A pressure vent cap is used on the radiator. The cap contains both a pressure relief valve and a vacuum relief valve. The pressure valve is secured against its seat by a spring of predetermined strength. The spring protects the cooling system by relieving the pressure when it rises above 90 kPa (13 psi). The vacuum valve is secured against its seat by a light spring which allows the valve to open in order to relieve the vacuum created in the cooling system. A see-through coolant reservoir is connected to the radiator by a rubber hose. The pressure/vacuum valve radiator cap allows two-way travel of coolant between the radiator and coolant reservoir.