Strip the brush and give the needle and nozzle a good cleaning. You likely have a paint buildup on the needle and/or the nozzle. Tolerances are so tight that even a microscopic layer of paint can cause the airflow to blow back into the cup.
1) Badger's online store presence, USA Airbrush Supply, is having a memorial day sale on brushes, parts and paint. 30% off through June 1. Replacing the needle and nozzle should fix you right up if the one currently in the brush is damaged or beyond service. I have ordered through them in the past and they are good to work with.
2) Badger has a lifetime warranty on all their brushes. Labor is free and parts are discounted, I want to say 60% off retail. You essentially pay shipping both ways