I set up to do some airbrushing today and discovered a pretty weird problem. When I depress the airbrush trigger straight down for air, it does not shut off when I release the trigger. Air then blows constantly out of the brush. I disassembled the brush until I had only the air valve itself hooked to an air supply. When I actuated it then, it worked fine. I tested it again at various stages during reassembly, and it worked fine each time until the front end is completely assembled, i.e. head, needle tip cone, needle. Then the problem recurred. I pressed the trigger again, released it, watched it continue to blow air, and then I slowly unthreaded the head from the front end of the brush. As soon as I did that, airflow stopped and the trigger worked normally. So for some reason when the head/needle tip assembly is tightened completely, it is preventing the air valve from closing when the trigger is released.
Has anyone got any ideas or seen this before? I'm stumped. 
Chris