Which of the following factors may result in decrease in the level of the cooling system liquid?

The engine cooling system is a pressurised and sealed system, which means that under normal operating conditions, there should be no significant loss of coolant. A rapid or noticeable drop in the coolant level in the expansion tank, shown in the photo, is most often caused by a leak. It can occur on components such as the radiator, rubber hoses or the water pump.