Yes, you should only get air. Check to make sure that the trigger is all the way forward and that something is not blocking it. I'm not familiar with the 155, I have the 150, does it have a set screw on the trigger to help keep a consistant flow rate? It will be a tiny little screw at the front of the trigger.
Or what Scott said! He's probably forgotten more about airbrushes than I will ever know!
![Wink [;)]](/FSM/CS/emoticons/icon_smile_wink.gif)