AB paint flow without pulling the trigger

I read this from the Eclipse HP-CS manual. I don't know if it will solve your problem. But it's worth a try.

To ensure smooth main lever action, lubricate the needle (#17) and the main lever mechanism (#12) regularly.

1. Unscrew the handle (#19).

2. Loosen the needle chucking nut (#18).

3. Remove the needle.

4. Coat the needle lightly with a high-quality lubricant, like Medea Super Lube.

5. Wipe the needle with a soft, clean cloth, leaving a light coat of lube behind.

Drip one drop of lube into the slot directly behind the main lever and as close to the main lever as possible. DO NOT over-lube the needle or the main lever mechanism, since it is possible to transfer excess lube into the nozzle causing severe paint-flow problems.

DO NOT use light machine oil or WD-40 for lubrication.

These will cause the needle to stick as it moves through the needle packing o-ring.

CAUTION: If it becomes absolutely necessary to dismantle the airbrush remember that in most situations tools are not needed. DO NOT use pliers. If needed, use the provided wrench to unscrew the head cap (#3).

Good luck cleaning your AB.

The needle is not seated properly or the nozzle is split.  I know nothing about Iwata brushes so I can't tell you how to loosen it, but you need to loosen the needle chuck and seat it in the nozzle correctly.  It isn't closing the nozzle off properly.  If that doesn't help check for a split nozzle.

To seat the needle on Badger brushes you just loosen the needle chuck, tap the back of the needle very lightly with a finger, and then tighten the chuck again.