A radiator cap contains a pressure valve that pressurizes the coolant.
A pressure valve opens and sends coolant back to the reservoir tank when the radiator pressure is increased.
A vacuum valve opens to discharge the coolant from the reservoir tank when the radiator is decompressed.
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[TD="width: 250"] Reservoir Tank
[TD] The reservoir tank is connected to the radiator in order to store the overflowed coolant from the radiator, and prevent it flowing outside.
As the temperature of the coolant in the radiator increases, it expands and flows into the reservoir tank. As the radiator cools, it draws up the coolant from the reservoir tank.