Hi,
I am using Mac version of ZBrush 4R5, the Alt key doesn’t work as expected in some brushes. Eg. in BumpBrush, when I drag with Alt key in Zadd mode, it won’t change to Zsub. On the other hand, when I’m in Zsub mode, Alt key won’t change to Zadd.
It just happened in certain brushes, such as BumpBrush and PaintBrush, but not all. How can I fix it? thanks.
As far as I can tell, that has been the behaviour at least since version 3.5. I’ve no idea why the toggle shouldn’t work for those.
However, it is not difficult to come up with an alternative. The attached macro will do the trick.
Save the file to your ZStartup/Macros/Misc folder. Restart ZBrush or press Macro>Reload All Macros and you should get a new button in the Macros>Misc section in the Macro palette. Assign a hotkey of your choice (though note you can’t use Alt on its own). Pressing the hotkey, or button, will toggle between Zadd & Zsub.
Thanks for the work-around 
Btw, do you think its a bug?
I’m also wondering if Windows version have the same problem…
I’ve really no idea. It has been there for a long time. Yes, it happens on Windows too.