Header Tank or Radiator Pressure Cap THIS IS A 15 LBS CAP 1. Seals the top of the radiator filler neck to prevent leakage. 2. Pressurizes system to raise boiling point of coolant. 3. Relieves excess pressure to protect against system damage. 4. In a closed system, it allows coolant flow into and from the coolant reservoir. The radiator cap pressure valve consists of a spring-loaded disc that contacts the filler neck. The spring pushes the valve into the neck to form a seal. Under pressure, the boiling point of water increases. Normally water boils at 212 degree F. However, for every pound of pressure increase, the boiling point goes up 3 degree F. If the engine overheats and the pressure exceeds the cap rating, the pressure valve opens. Excess pressure forces coolant out of the overflow tube and into the reservoir or onto the ground. This prevents high pressure from rupturing the radiator, gaskets, seals, or hoses. CAUTION Always remove the radiator cap slowly and carefully. Removing the radiator cap from a hot pressurized system can cause serious burns from escaping steam and coolant.