I've been fighting with zbrush for a while now trying to get the mysterious roller brush to work. The idea is that you can roll on alpha brushes like you would use a roller brush with paint on your walls at home. This is perfect for doing fabrics, skin pores and all kinds of cool stuff. Thing is i couldn't bloody do it. As it turns out it wasn't the roller brush i was after at all, but a little button in the stroke menu "roll" which modifies your current stroke (dots and freehand work). So bloody obvious! Works outside of projection master too.