The India ink I used to replace it needs a bit of thinning, rapidograph ink might work better? But I'm loving it. Pentel makes the best affordable synthetic brush heads IMO. And the more viscus india ink both runs better and it's permanent now when dry!
Here's a rough diagram of how i modified it, it's a very crude mod, but effective so far!
The Art brush is the same brush head essentially as on a pentel pocket brush, both tools I recommend. Not paid really! :) I wish.The Art brush is slightly longer and thicker, but can keep a very nice fine point! They also come in colours which can be cool. But the ink is super NOT permanent, and bleeds rather dramatically with any wet media.
The brush heads are fantastic, and their greater length gives them a slightly smoother line while pulling the stroke. Pentel's synthetic brushes are my hands down favorite, once you get the hang of them they give lines amazing personality.
Used properly they are also amazingly durable. They can get splits, but usually by either forcing against the base too much in strokes or accidents mashing it while putting the caps back on - done that once or twice. Ack.
But i've got a pocket brush that's over 5 years old and still holds a perfect point, and the modified art brush with india ink is currently my favourte tool! Been using the pocket brushes since 1996 or so? When I had to use them on a job at Nelvana studios in Toronto. I fell in love. Love love love.