Urich,
There is probably paint fouling the valve under the trigger. I have this problem with one of mine and the only thing I can recommend is cleaning it more. I'd be real careful about using acid or paint stripper especially around the valve at the trigger. I'd recommend having it serviced rather than risk ruining the brush completely. In my case, paint flowed back to the valve area after the packing that is supposed to prevent paint from going there was damaged. ( A good lesson in why you should clean the brush after every session, rather than letting paint build up and dry.) Your Iwata may need new packing also.
Iwata-Medea offers service. Call 503-253-7308 ext 10 for a return authorization number and instructions.
Cheers, Aaron