A leaking coolant expansion tank is an extremely common problem in cars over 10 years old. That is because the function of the tank is to allow for the coolant fluid to expand as the operating temperature increases. Over time, the coolant pressure takes its toll on the plastic tank and a crack will usually develop. Most times it happens near the bottom of the tank where the low fluid sensor or hose connection is located. You will usually be able to tell you have a leaking cooling reservoir by looking under the engine compartment while your car is parked with the engine running. If you notice a puddle of green/yellow fluid, then you've probably got a cracked tank. This means that you are running low levels of coolant through your engine's cooling system; which can lead to overheating and a cracked engine block or cylinder head. So, clearly, replacing a damaged or broken coolant recovery tank is always in your best interests.
Liquid cooling became popular on internal combustion engines after WWII. Early designs, however, did not always include a coolant reservoir. The problem this causes is a build-up of pressure when the temperature rises, so the response to this issue was to add a tank where the expanding coolant had a place to reside. This is the coolant expansion tank (or coolant recovery tank). It is made of a very durable heat-resistant plastic, a cap that is rated to withstand between 15 to 20psi, and usually includes a fluid level sensor at the bottom. This will warn the driver when the fluid is running low.
Changing a coolant reservoir is a very straight-forward procedure, and doesn't require special tools or an advanced ASE certification. All you have to do is perform a few simple steps.