I do NOT recommend soaking a lever filler pen in water, not even COLD water.
Why, because of RUST. Try as you might you will NOT be able to get all the water out of the pen. And where the water is in contact with iron/steel (lever, C-ring, J-bar, or other mechanism), it will start to rust. If you want to clean the inside of the barrel, do it dry, with a cotton swab or 'rat tail' brush.
The cap, in general, I would use lots of damp cotton swabs only in the inner cap. With some caps, you don't want to get water between the inner and outer cap. Because water will start to rust the part of the clip that is inside the cap.
There is also one pen material that I understand will soften in water. So again, dry cleaning. Better safe than sorry.
Do a LOT of research, and ask questions.
There is a LOT of bad info out on the internet, and following some of those could cause you to break a pen. Just because it worked for the guy doing the YouTube video, does not mean that it will work for you, or others. Been there, done that, became very upset.
Edited by ac12, 21 April 2016 - 03:55.