Hi Just found a fix for my adjustment brush shutting me down...
In a post from Victoria Brampton:
Re: Lightroom adjustment brushes quit working
Christina, check the Flow and Density sliders aren't set to '.
I had my density slider at 100. I put both flow and density to 50 and my brush is working.
This must be a known issue as the fix was posted 17 Nov 2009, 2:42 pm, odd that it has not been mentioned before.
Worked for me hope it works for you.