Though you may have done this...have you completely taken the brush apart and made sure that any paint build up has been removed from the nozzle and the needle? The blow back you're describing sound similar to when intentionally block the nozzle - as when you clean the thing
Take the nozzle off and clean is completely. While you're at it, take the needle out and make sure that any paint build up is removed from the tip and on the section near the trigger. If you run two of your fingers along the needle, you may feel some build up. If so, get some thinner and make sure the needle is completely clean and smooth.
I clean my brushes completely after every session. I suspect that the smaller the orifice, the more prone an AB may be to the problem you're describing if is isn't cleaned well. I know that you said you have cleaned the AB, however, I am amazed at the amout of paint it areas of an AB that I find when I do a complete break down for cleaning.
I don't know what size needle you have, but my smallest is .15 mm. I don't think it takes much build up of paint anywhere to mess with that small of an orifice.