I just replaced my old Waterpump this past week. I have only driven about 40 miles total since replacing the pump. Every morning the water level in the expansion tank has dropped 1/2". I fill it exactly between the 2 lines on the cold fill mark, in the morning it's back to the lower of the 2 lines. Add a cup of distilled water and its back to the middle. Drive it 5-10 miles....back to the lower line the next morning. Just getting the air out? I've looked and looked for a leak with no success.