With flexible tubing over time some of the water will permeate the tubing and evaporate. So you will have to top up from time to time. Different tubing has different rates of permeation. It is a slow process, so you may have to top it off after 6 months or so. This is why I like to fill my system to the top. Then after a week of running it, top it off after the bleeding process is done. Then I know that I am good for a long time.