I have a similar model Iwata, bought it in November to upgrade my workhorse brush, but haven't really used it yet because the instructions that come with the brush are mainly in Japanese, with a Japanglish translation. And the instructions tell me not to remove a mystery part, not identified in the exploded view.
If you could explain your usual cleaning procedure for your brush, that would be very helpful. Apparently its very easy to clean the Iwata. But with the fixed paint cup, it doesn't field strip like my other brush, so I'm not sure how to get at the internals, or if its necessary.