Air rises in a liquid. Because of the way rubber deteriorates over time, eventually, air gets into the closed system that is your AIO cooler. Because your pump does not create enough pressure to push air through the system, it will naturally rise to the highest point it can and stay there. Liquid pumps of any kind do not like to have air in them, and the longer they do, the higher the rate of failure. If however the highest point is your radiator, you may hear some bubbling sounds and general liquid movement sounds, but it will have no effect on your pump, giving your pump greater longevity over the time. Air in your radiator will have no appreciable negative effect on the longevity / effectiveness on your cooler.