I found this to be a way around this issue:
Open the image that you want to draw on, keep that in its own RGB window.
Open a brand new document, CMYK.
Activate your Brush and draw a few lines on your artboard.
Then, copy and paste your RGB image into this new CMYK color space.
Voila. I think the Brush can only be used in a CMYK color space. And sometimes this does not work when you convert your document to a color space inside the RGB document.
Hope this helps. If anyone else has an alternative, let me know.
I've always had this issue with the Brush.
Weird. My problem was that I needed to also choose the brush as the same
size that was also the brush chosen on the tools bar. I believe I got the
answer from someone in your forum some months hack.
On Sun, Nov 25, 2012 at 10:06 AM, StevefromFierce
Load a brush first. That way you will be able to use the paintbrush tool. I usaly open the Brushes palette first, then click on its menu and select Open Brush Library to be able to select the brush I want.