My CRV has a tank attached to the radiator. According to the handbook it is a coolant reserve tank. I have owned this car for a year and paid no attention to this tank as I thought it was just to hold the overflow from the radiator. The level was always around half. Today I noticed the level has dropped and on closer inspection I see there is a minute and Max mark on the bottle. Mine is now sitting close to min. Does anyone know what the purpose of this tank is and should I top it up?