Basic Stuff
Here is a sketch I made to illustrate the basic principle of an internal mix airbrush. This represents the head assembly of a typical internal mix brush. The needle is centered in the nozzle or tip, and is surrounded by paint. Moving the needle forward or back changes the size of the opening at the front and the amount of paint that is sprayed. Surrounding the tip is the spray regulator, which is made so that pressurized air can exit in a circular opening around the tip. The air escaping creates a vacuum that draws paint out of the tip and down the needle to break free at the point and form a spray pattern. The thing that makes this work is called the Bernoulli principal or Venturi effect. If you're curious, do a Google on either of these words and you will find all kinds of information.
Note: if the nozzle doesn't extend a tiny bit past the spray regulator, there will be pressure rather than a vacuum and air will be forced back into the paint path. A common symptom is bubbles in the cup or jar.
In a single action brush, the needle is moved by turning a threaded shaft. In a double action it is moved by pulling back the trigger. In either type, air is controlled on or off by pressing the trigger.
The sketch is crude. The actual devices are very precise.
Here is a schematic drawing to try to show how a typical gravity fed dual action airbrush works.
Paint is held in a cup on top of the airbrush and is prevented from entering the rear of the brush by the needle seal, which is sometimes called a needle bearing. The needle slides through the seal and passes through holes in the trigger, the rocker, and the needle tube. The rocker is sometimes called a back lever. The needle is locked to the needle tube by the needle chuck. There is a spring around the needle tube that forces the trigger all the way forward.
The needle is positioned so that when the trigger is all the way forward, the needle is fully seated in the nozzle, sealing off the paint exit path. Pulling back on the trigger causes the needle to retract and allows paint to flow out the nozzle. Pushing down on the trigger opens the air valve and allows air to escape the nozzle and create a Venturi effect that draws paint down the needle tip and spray in a conical pattern.
A siphon, or bottom fed airbrush works the same way except that the vacuum draws paint up from below.
Here are some details of typical airbrushes. The Testors Aztek is very different. You can look at the Aztek review to see what it's like.
First, a cutaway image that I borrowed from Badger of the Model 100.
Notice the "O" ring in the air valve assembly. It will swell if exposed to lacquer thinner. Also notice that the needle bearing prevents paint from leaking into the rear portion of the brush. As long as the needle bearing is in good shape, the rear portion should never need cleaning. Also, notice the spring screw just in front of the needle chuck. This can be adjusted to change the force needed to pull back the trigger. The needle chuck is what secures the needle to the trigger mechanism. Loosening it allows the needle to be removed for cleaning. With the trigger all the way forward as shown, the needle should be positioned all the way forward too, so that it just closes off the opening in the tip
Here is an illustration from Charles Burdick's 1896 patent for an airbrush. Not a lot has changed in over 100 years.
Here are some samples of the types of head assemblies you might encounter.
First is a picture of the head assembly of an old style Badger 200. It's just like the ones in the model 100 and 150.
The tip is a tiny brass piece that screws into the head. Normally, you can leave it there. The tip and head can be replaced as a unit. The head has flats for a wrench that is used to tighten it. The Teflon head washer can wear out, and when it does, it can cause bubbles in the paint cup. It might be a good idea to leave the head a little loose when the brush is put away.
Here is a picture of the head assembly of the Badger 105 Patriot. The 155 Anthem head looks the same.
There is no Teflon head washer, and the part labeled TIP is free floating and self aligning. It is held in place by being captured between the head and the main body of the brush. The only parts of the brush that come in contact with paint are the tip, the needle, and the main body. The head and regulator should never need cleaning. This is a newer design and is found in many airbrushes. Note, if the tip doesn't seat right, air can leak into the paint path and cause bubbles in the paint cup. The head and regulator should be finger tightened in this model. No wrench is required.
Here is the head assembly of the Master G-22 airbrush.
This type has a very tiny tip that screws directly into the body of the airbrush. They are very fragile. The brush can be used without the end cap. It is only there to protect the needle from damage.
Badger makes two types of trigger mechanisms for their dual action airbrushes.
The type on the bottom is used in the Thayer & Chandler Omni and Vegas, and the Badger Renegade and 3155 airbrushes. The upper type is used on all other models.
I see no real advantage of either one over the other. They feel about the same to me. The upper type is a little more difficult to re-assemble, but you rarely need to take it apart in the first place. Its needle tube has a flat surface that must align with a D shaped hole in the tube shank. The main thing you need to know is that the tube shank of the upper type must be tightened all the way. Otherwise, the whole assembly will turn when you loosen or tighten the needle chuck The spring screw can be set as desired to adjust spring tension. In the lower type, the needle tube is kept from rotating by its attachment to the rocker, which slides in the trigger slot. The spring adjuster does just what it says, and can be set any way you like.
The Master G22 has a different type which is fairly common.
It's kind of a combination of the previous two. The rocker is separate, but the needle tube is kept from rotating by a groove that rides on a screw in the body of the brush. Notice that the trigger is two parts. The bottom part pivots and fits in a hole that leads to the air valve.