My favourite media for urban sketching is brush-pen (Inktense pencils a close second), and my favourite brush-pen is the Kuretake #8. It’s the cheapest (direct from Japan here), has great ink-flow (the Pentel is very dry), the brush is springy (firmer than the expensive sable version) and very fine-tipped, and it lasts well. But it can be improved in three ways…
It’s a bit long for pockets, so I cut the end off the barrel and glue a piece of wooden dowel (left over from building flatpack furniture) in the end. 
It lacks a clip on the cap, so I take one off an old Pigma pen, slightly widen it with pliers, and glue that in place too.
And the ink it comes with isn’t waterproof (and isn’t grey!). So I empty the cartridge and reload it with Lexington grey.
Now it is near perfect! 
For more on brush-pens see this page on this blog, or this highly comprehensive one from JetPens.
Mostly drawing 
What excellent hacks! I have several brush pens I love, but I haven’t tried this one. Will have to look it up.
Tina
Hi Tina, I was very pleased that my favourite brush-pen’s also the cheapest!
Ed, what are those scratch marks on your Lamy? Is it for you to recognize what it’s loaded with?
Well spotted! When the cap’s ‘posted’ on the end of the barrel there are 3 scratches, going right round the pen, that divide its length into quarters. This gives me a handy way to gauge distances etc in a complicated sketch by holding the pen up against the view. It sort of works, and I’ve done similar vandalism on my other two Lamy’s, but not as crudely! Which reminds me to do a similar piece of work on my brush-pen, this time possibly into thirds so it’ll match the acetate grid I sometimes use.
Hello! I’m not sure if you are still active and I hope you’re doing well. I have a question. Its for another brush pen but this is the closest post for modifying one so please excuese me if its inept. Is there a way to reduce ink-water flow of pentel aquash brush pens? I sketch and spend a lot of papers so I cant afford thicker papers and also cant buy normal brush pens with that variety of tips too because I’m in a third world country. Is there a way to apply pentel colors brush pens tube influence to aqush brush pens?
To reduce flow maybe put something very thin (a pin tip?) in the valve part of the water brush to restrict the flow?