By default the brush tool is in "add" mode. If you hold down the ALT key (not sure what the Mac key is), it will put the brush into "subtract" mode. Both modes remember their own sizes and feather amounts.
Thanks for your reply.
I am aware of what the brush does.
What I would like is the ability to having
automasked something in the foreground, to invert
the mask and adjust the background, pretty well
like the elliptical tool. Photoshop type
selection tools would be terrific, but a
rectangular tool would suffice. It would also be
good to be able to erase parts of a clone and to
adjust a clone for tonality to get a better fit.
What you are looking for is beyond the abilities of the "picture editor" in LR. You are looking for an "pixel editor" like Photoshop or PaintShop Pro.
There are a lot of people now who exclusively
work in applications like Lightroom. The want
non destructive editing with all the tools that
applications like Photoshop use. Lightroom is a
pixel editor that uses low res proxies to do its
work and as far as I can see it should not be a
great problem to incorporate Photoshop like
selection tools. Photoshop uses smart previews,
which are low res proxies. It will happen,
because they will have trouble selling Lightroom
unless Adobe adds better cloning and better
selection tools for local corrections. If I don't
upgrade my camera, I will not need to upgrade
Lightroom 6, but if they add better tools as
described, so I can make more detailed
corrections, it gives me an incentive to upgrade.
I have stopped upgrading Photoshop as I have no
need for anything better than CS5. It is going that way now with Lightroom.