I have an overflow tank with my radiator and I am a little confused. After I run the truck for a while the antifreeze flows into the overflow the way it should. The problem is after the truck cools down it seems to pull more antifreeze than it should. I fill the radiator to the bottom of the neck and after it cools down the level is up to the top of the neck. If I fill it to the top of the neck it seems to still pull more from the overflow tank then it should because there are drops of antifreeze between the fins. I had a pressure test down at a radiator shop on Monday and it held pressure and check out good. What am I doing wrong?