Hi Dan!
Thanks a ton for your help! I've uploaded a .afbrushes file with two brushes. The first one is the one I open this thread about: does not paint anything at all with the WACOM pen in 1.9, but it does paint if using the mouse. In 1.8.5 it works. I've also tried in some 1.9 betas, and it does not work there either.
I've added a second brush whose drawing is completely different between 1.8.5 and 1.9. In this case, it draws, but the effect is completely different.
I'm a SW developer, so if there is anything that may help you (trying some patches, debugging in my computer, hex-edit some files a few times until I find something working, etc) just tell me!
Thanks a ton again for all your help, Dan.