Hud - if you are just getting the "I've done all I can do with the brush" feeling. There are several good brushes which are an upgrade to the one you have but won't break the bank:
Siphon fed: The Badger 155 will handle large coverage to medium detail just fine! The Iwata HP-CR is also a good brush, can handle large coverage to a little bit finer detail just fine.
Gravity fed: Badger 100LG with a medium and fine needle/nozzle. With both tips this brush can do a lot! Iwata HP-CR it can handle almost every job, I own this one plus the Badger 155 and this is my favorite...
All of the above brushes typically run roughly about $70 plus the airhose. (Badger and Iwata have different size connections on them, neither of which will work with an Aztek hose)