Go to the Testor/Aztek website and send them an email describing your problem. My expereince has always been they will provide instructions on either fixing or replacing the brush. My expereinces with them have always been very good. They are fast, polite and don't try to nickel and dime you to death. They have repaired several airbrushes for me. In each instance, the damage was caused by user misuse - not necessarily mine.
I have never had many problems with the Aztel airbrushes. When cared for properly they have provided years of good solid use. I did send one back after I used Testors paint remover to clean the body (didn't read the can - paint remover is bad mojo for nylon/rubber bushings in the body). They fixed the body for me.
I was given another Aztek by a fellow modeler who just couldn't get it to work right. It turned out the body was filled with dried paint (how he got that much paint in the body is beyond my comprehension). they fixed or replaced it, no questions asked. Works great now.
The Aztek allows me to do whatever painting I need - even fine camo. I have inherited several sets that friends pretty much gave me because they couldn't figure it out. It ain't rocket science. I have found it easy to maintain, only takes seconds to clean thoroughly and is flexible enough for any job. it's also durable, and is backed by the best customer service I've come across in this hobby.
I recommend them whenever I can, but often get scoffed at by those guys with high tech brushes. But I'm usually spraying while they are cleaning, or fiddling, or thinning paint yet again.