Try createx airbrush restorer. Made for just that purpose and works well. At least it cleaned up the mess I made of my airbrush a while back
Also pick up a pack of airbrush cleaning brushes and use those after soaking in the restorer as recommended. I also use the brushes when cleaning the airbrush after a painting session. Helps get all the crap out of the brush that tends to build up over time.
You have to be careful about soaking your airbrush in various substances cause you can damage O-rings or remove oil inside the brush. A few other things can be damaged too, at least that is what I read somewhere else.
Jigman