Is there a kind of mousedown with the brush tool?

[Tiemo Hollmann TB]

Hello,

when using the brush tool no mousedown or mouseup messages are send. I
didn't find any hints in the docs for other messages. (rawkeydown doesn't
work either)           

Does anybody knows how to intercept "mousedown" with the brush tool? I
wanted to use it for my undo function, to grab the state of the image before
painting.

Thanks for any ideas

Tiemo