Poor Pentel brush pen. It didn’t do anything wrong. I just wanted to see if I could use fountain pen ink in it.
I took off the white ring as instructed. Then I removed the stopper from the ink cartridge. (The cartridge is also the body.) The stopper isn’t meant to be removed; it’s there to regulate the flow of ink from the body to the brush.
It took some time to pump and rinse out the ink from the cartridge. I wasn’t that successful – while I squeezed the cartridge under water 23,954 times, black-tinted water still came out. (Squeezing out the ink isn’t a straightforward operation. The flow is very controlled, and there seems to be a second layer of material inside the cartridge.) I squeezed it one last time, plunged it into a bottle of Pilot Iroshizuku tsutsuji ink, and wondered what the result would be.
(Yes, the tsutsuji stained the paper label on the brush pen. It looks like rivers of fuchsia lava underneath black rock.)
Tsutsuji (fuchsia) + excess water + remnants of black ink = a serious, don’t-mess-with-me purple. As the excess water evaporates from the brush tip, I expect the lines to be not as watercolor-y, but more solid. I’m happy with this hack, and hope for non-bamboo doodles to come.