Cleaning the coolant expansion tank on your car may be necessary after a few years. The inside will definitely get dirty and some deposits may form around the coolant level line. There are a few ways to clean the inside of and you may not have to take the tank apart if it not too dirty.
Cleaning the Tank in Place
Make sure to wait until the car is cool to avoid getting burned with the liquid inside the coolant expansion tank. Disconnect the hose under the tank to allow the liquid to come out. Stick a rag into the hole and poor a mix of mild bleach and water all the way to the top. You can let it sit for a while and use an old toothbrush to brush the inside. Then remove the rag and use water from a hose with a little pressure to rinse it out.
Taking the Tank Apart
If the tank is real dirty and the above steps won't get it clean, disconnect all the hoses from it and take it out. A good trick is to fill it again with bleach and water and ice cubes. Shake the tank vigorously to clean the inside. Repeat a few times and rinse it with the hose with high pressure.