being you disassembled it, this is probably moot, but make sure the nozzle tip (head, I think badger refers to it in the parts diagram) is down tight, if it is loose it will either not spray paint or allow air bubbles into the paint... just grab the serrated ring at the end of the airbrush and with the front of the AB facing away from you turn it to the left, finger tight....
I feel it is more what scott said, being a used airbrush especially.. but thought I would cover that base as well...