You should regularly check your coolant levels. If it is low level, add coolant until it is in the right level. If it is your schedule to rplace coolant, then drain the old coolant. Flush with distilled water until clean, then replace the coolant. The reservoir innolder cars is a an overflow tank. As the coolant heats, it expands and builds pressure then overflows to the reservoir tank. When the engine coold down, low pressure sucks it back to the radiator. Never fill the reservoir to the top, there should be space left for the overflow.

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