Coolant in a sealed system does not evaporate to the atmosphere, and should never need topping up.
Instead, a pressure cap is fitted to the system, and a tube leads from the top of the radiator to a glass, plastic or metal tank or container.
Coolant which has expanded as the system warms up passes down the tube into this tank, and is drawn back into the radiator when it has cooled.
The system is supposed to be maintenance-free, and is drained only when a hose or the coolant is due for replacement.
When draining and refilling the system, if the coolant is to be reused, place clean containers beneath the drain points.
Note that the coolant is poisonous. Do not use containers which will be used in or near the kitchen. If you do not want to use the coolant again, do not pour it down a drain — ask your local council how to dispose of it.
If you intend to reuse the coolant, have a little extra to hand for replacement, as a little spillage is inevitable.
Most manufacturers recommend a 60 to 40 per cent water and antifreeze mixture. Some specify a 50-50 mix.
Use antifreeze made to BS (British Standard) 3152, or as specified by the car manufacturer. The standard number, if it applies, is printed on the container.