Waterbrushes were designed to hold water so that when you are outdoors or on the go, you can easily dip the brush in watercolour and start painting. However, I've heard professional artists say that it isn't the best thing to use with watercolour because it expels too much water. Or you can't control the amount of water to paint ratio very well. I found that it is true. So I stopped using it for a time. Then, I found that some artists around the world use it in a different way. And it works well.
Instead of filling the waterbrush with water and dipping it in colour, you put the colour into the waterbrush! While that limits the waterbrush to only one colour, you get a constant flat colour with decent flow when painting. Not being able to control the flow isn't a problem anymore because the intensity of the paint is always constant. If you like, you could fill a few waterbrushes with a different colour each. Afterall, a limited colour palette and a good control of tonal values can produce nice illustrations too. You don't always need a rainbow of colours. Here are some quick illustrations I've done with it.
The waterbrush I use is by Pentel. For colour, I use ecoline ink (with is liquid watercolour). I like my colours a little desaturated so I add water to dilute the intensity of the ink. And I also add a few drops of black ink to darken it. It's really up to you. You can also use fountain pen ink. A syringe with a fat needle is a good way to fill your waterbrush.
It was fun using it this way until it started to clog and it stopped flowing. You see, when you use inks or watercolour and put it in a waterbrush, it may form sediments which can clogged its small channel. I use to think that that was the end of the waterbrush and so buy a new one. But really, it is a small problem.
Just unscrew the top half off.
Get pliers with teeth and pull on the...thing. I don't know what to call it. haha.
It might slip off a few times. But grab it tight and pull and it will come out. (It's a snap-into-place kind of piece.) It's ok if the surface becomes frayed.
This is the part that gets clogged. Just put a sharp object through it to clear it. Some waterbrushes have a small white plastic bit inside (to slow down the water flow). You can push it out and put it back in. If you take it out, however, your waterbrush will flow more freely,
That's about it. But it if you want to do a more thorough cleaning, you can grab the brush with your fingers and push it out the back.
Wash everything under a tap and put it all back the way you took it out.
And that's it! Have fun with your waterbrush and don't throw it away if it gets clogged!