If I remember right, the expansion tank is the where the radiator cap is. Right next to the cap is a small plastic bleeder screw that a quarter fits into easily to open it for bleeding. It is part of the radiator. If you have opened the radiator, you may have to bleed the system now. This is pretty time consuming since you have to do it many times to get all the air out. I know this because I had to put a new head on my car because I was low on antifreeze....
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