As the pressure exceeds , the set limit of system , pressure cap starts functioning and lower sealing discs is lifted releasing pressure and allowing the hot coolant to escape through the overflow tube provided in the fillerneck . If single sealing cap is there and no reservoir is in the system, then the coolant will be lost . However , if double sealing cap is there and the recovery reservoir is provided in the cooling system , then coolant escaped from overflow tube is collected in recovery reservoir and cooled down the temperature.
When the radiator is cooled, coolant inlet temperature reaches below boiling temperature , it contracts and the pressure inside the radiator reduces and reaches to negative pressure i.e vacuum is not broken , it damages the radiator and its hoses in extreme cases . The vacuum valve provided in the cap comes in operation and allows Air to enter into the radiator via overflow tube of fillerneck in case of single sealing , and coolant from recovery reservoir in case of double sealing and breaks the vacuum in the cooling system.