I will admit that I am one of the writers who prefer non-cartridge/converter filled pens. In fact, when I get a CC filler I test it with water to see if it will function as an eye dropper filler.
With that said, after experimentation I have overcome the issue with ink "hanging" up in the converter, a trick I learned actually from Frank Dubiel many years ago. Assuming that your ink flow channel in the feed and the nib are set correctly and are clean, rinse the section very well and then soak the section in a solution of cool water with a drop or two of ammonia based household detergent. The soaking will ensure that any minuscule ink particles dissolve. If you don't feel comfortable with using ammonia then just use a good detergent.
Usually no more than an hour or so is necessary. Rinse the section well again and use a paper towel or tissue to wick the excess water from the nib/feed. If it appears very clean then you should be good to go.
In a clean solution of a few ounces of water and a drop of household detergent, clean the piston converter well. If you've flushed your section then it's not necessary to draw your cleaning solution up through the converter unless you're concerned about the converter tip/port for the feed still being a bit cruddy.
If your converter can be disassembled, that would be a good thing to do every once in awhile. It also doesn't hurt to use a tad of silicone solution on the converter piston head. When I say a tad, I mean using a frayed toothpick with just a touch of solution.
DON'T flush the converter with clean water. Let the bit of soap residue remain in the converter. Reattach to the section and draw in your ink and expel a few times. Finally, crank down on the converter fill knob a tad until you can see a drop of ink form at the nib. This will ensure that you have good capillary flow, which, theoretically, will be aided by the slight soap film in the converter. The soap will keep ink from "clinging" to the sides of the converter at the molecular level, breaking the surface tension of the ink.
I use this same procedure for some of my finicky piston or ED fillers, sans the removal of the section part. Other than casein, you can safely soak most sections in water. Note that detergents with ammonia bases will stain hard rubber if you soak for too long. But cleaning vintage pens is another subject.
As a final note some folks like to use Koh-I-Noor solution to clean their pens. That's perfectly fine but with a bit of research you'll find that K-I-N is actually just a fancy soap. Most of the solution is TEA (not that you drink silly, it's triethanoimine) which is an ammonia type organic substance. I don't know the exact ratio of ammonia and ethylene oxide, but the substance is a base and in its pure form I think the pH is around 8.5 or so.
This makes TEA a good surfactant (reducing the surface tension of a solution) and emulsifier, which keeps the main compounds in a stable solution.
There's also a tad bit of potassium hydroxide, which acts as a buffering agent to help keep the pH stable even when the solution is picking up junk from your pen.
So what have I just described folks? SOAP. So IMHO make your own solution and save the money for pens :-)
I hope this helps a few folks that like to experiment. Best and blessings to all.