It should be replaced as soon as is convenient because excessive cooling leads to increased engine wear. If this happens the thermostat must replaced as soon as it is safe to do so. An older vehicle probably has a brass radiator. The end caps are soldered on and can be removed by a radiator specialist for inspection and cleaning of the inside of the cooling tubes. A modern vehicle probably has an aluminium radiator core with plastic end tanks.
Water expands when it gets hot.
To prevent loss of coolant, an overflow tank collects any driven out of the main system by expansion. Coolant from the overflow tank is returned to the main system when the engine cools down. In some systems the overflow tank is not pressurized. Here the main-radiator cap only allows water out under pressure into the overflow tank, but offers little resistance to its return. The cap must have a good lip-seal to stop air being drawn back in instead.