Radiator Cap : This cap is actually a pressure release valve for the radiator. When the fluid in the radiator heats up it expands causing the pressure to build. The radiator cap is used so that the pressure can escape. The setting of the spring tension on the radiator cap determines the maximum pressure. When the pressure reaches its maximum, the pressure pushes the valve open, allowing the fluid to escape into the overflow tank. This keeps air out of the system. When cooled, the system opens another spring that allows the fluid back into the system. It is recommended to replace the radiator cap when replacing the radiator.