The coolant overflow tank should never be totally full. Check the coolant level when the vehicle is cold. If the coolant level sits between the maximum and minimum level marks, there’s no need to add further coolant. If the coolant level is below the minimum level, top up the coolant using a mixture of the correct coolant for your vehicle and distilled water.
A Natrad specialist can help you to find the right coolant for your vehicle. If you think you have a coolant leak, have your vehicle checked by a technician to determine the cause of the leak.