How the expansion tank works.

As temperature of coolant rises, the coolant expands which leads to increase in the volume and pressure. This is the reason why there is low and high coolant level mark on the expansion tank..
There has to be a way to relieve this pressure. This is the importance of the cooling system expansion tank. The expansion tank catches the excess pressure and vent it through the radiator cap (In this case, the expansion tank cap).

Effect of coolant reservoir failure
1. If the expansion tank fails to relieve the pressure in the cooling system because the reservoir cap is faulty and cannot vent the pressure, it causes failure of the tank itself (by cracking) or failure of other cooling system components like hoses, plastics or the radiator itself.

2. Another way through which expansion tank failure affects the cooling system is through crack or breakage of the expansion tank that cause it not to be able to keep the cooling system at optimum pressure. If there is no sufficient pressure in the cooling system, the coolant quickly boils.