Hello, my first post here

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I have Iwata HP-B, which I was using for the last few years. Mainly with water based acrylics, very rarely with solvent based paints.
I've recently noticed two problems with it: air flow seems to be restricted (I set regulator at 20psi, but air comes out as if I had it at say... 10psi), and trigger has a definite sticky / spongy feeling. I stripped the a/b and cleaned it as well as I could; the trigger got better, but the air flow did not. I can see some old paint / gunk in the body (both in the trigger area and the front slit, where the air comes out) and wanted to soak it it some sort of cleaner.
What substance can I use for such task? I don't want to do any damage.
Also: I plan on soaking only the body, without nozzle, needle, air valve, etc. (I cleaned them well already). The only thing I can't seem to take out is the o-ring sitting behind the paint cup, separating front of the body from the rear. Would soaking the body with the o-ring in place damage it? Or maybe: how do I take it out to not to have to worry about it?
Any advice appreciated,
Chris