Expansion Tank : On some cooling systems, a pressurized expansion tank is used. The expansion tank will have a radiator cap on the tank. If the cap is not a pressurized cap, then tank is just a coolant recovery bottle. Many expansion tanks have a coolant level sensor installed in the tank. This sensor will let the driver know that the coolant is low. Expansion tanks are usually made of plastic. They can get brittle and crack after they age. If an expansion tank leaks or is cracked, they can not be repaired, they must be replaced with a new expansion tank. The beauty of an expansion tank is that you can see the coolant level inside without having to remove the cap in most cases. There are some styles that are not see through, however, you should never remove the cap when the system is hot to avoid personal injury.