For small brushes I use interdental brushes that you can buy at Wal-mart or Walgreens or places like that near the floss and toothbrushes. They are small enough to go between your teeth so they fit nicely in the end of your gun. The ones I buy are on nice handles with wire shafts so you can bend them to go through the cup as well. They aren't long enough to go from the end all the way to the cup so I do them both ways. I generally dip them in Denatured Alcohol and brush away...I do the complete breakdown only once every couple of weeks though. I've got all kinds of guns including the PS900 and those brushes work on all of them so far.
They are only about $3 for 8 of them and you can use them a bunch of times. Its the only thing I've found that works to clean that space easily.