Andre,
I am assuming that what is happening is that paint is dripping out the tip even when there is no air? You mention that this problem happens only in single action mode. I am not familar with the A470 single action mode. Does the single action adjustment hold the needle open to a desired preset position? If so, I suspect that maybe the side feed color cup is not appropriate for single action use. The color cup sits above the level of the tip, so the paint is feeding to the tip by force of gravity. If the needle is open, I would expect the paint to dribble out the tip.
If the above makes sense and seems to be accurate with respect to your problem, you may have to use the siphon bottles when using the AB in single action mode, or use it as a double action when using the color cup.
If all of our suggestions seem totally wrong, send an e-mail to their customer service. I don't have the address handy, but when my A430 went kaput, I went to www.testors.com, and funbled through Testors website, until I could find a customer service address. Their service representatives were quite friendly and helpful as they could be.
Good Luck (Bon Chance?)
Andy