Don't know why I had to keep adding coolant about every week. First I thought I should begin to replace all the water hoses since the truck has been about six years old and never been replaced. Well the problem still was not fixed. I took a good look on top and bottom of the engine and around the radiator in also the reserve bottle to see if I could see a leakage and was unsuccessful of finding a leak. So I took the truck to a good auto mechanic to see if he could find where the coolant could be going and he was unsuccessful too. However he told me he had worked at GM for a long time until he started his own shop and he told me that he heard that when you add coolant to try to add it first thing in the morning before you drive the vehicle or wait a long time for the engine to be completely cool because you tend to trap air in the bottle which when you add coolant and your engine gets warm the coolant will leak out the overflow. So I did this and it still did not help. So I went to a auto parts store and bought some stop leak and poured it in the reserve bottle and it seemed to help. Still I'm sure it will leak again some day. Still don't know what to do.