I inherited a rather old Iwata Revolution a while ago, which should be a big step up from my much more basic one. However, after some initial success and then a break, I'm struggling to get it to work.
Symptom: Pushing the trigger down, in any position, causes air to rush out of the nozzle as usual. Pulling the trigger back leads to a backflow of air into the cup, causing the paint to bubble, and no paint comes out of the end.
What I've tried: Complete disassembly and cleaning with lukewarm tap water. Attempted to spray both water and acrylic medium, but both exhibit the problem described above. Visual inspection found all parts appear clean, uncorroded and undamaged. Pulling the needle back by hand (instead of using the trigger) produces the same effect.
Background: The needle and nozzle are both quite new (I had to replace them). I had it working quite nicely early in the year but it's been unused for 2-3 months. It was thoroughly cleaned and disassembled before this break. I mostly used it for acrylics; I'm not sure how the previous owner used it but I believe he would have taken reasonable care of it.
Any ideas about what else I could look into?
UPDATE: On @fixer1234's advice I took it apart and gave it a 4-hour soak in hot water and dish soap, replacing the water with freshly-boiled water when it went cold. I then soaked all the parts except the main body in denatured alcohol for another couple of hours. No change unfortunately. I could repeat the latter process but the parts are already gleaming and it's hard to see what I'd be trying to achieve.