tankowner made two great points. Firstly, I don't think that your brakes would be a very wise testing ground for an experimental fluid. Secondly, of ALL the fluids in the modern automobile, brake fluid not only make up the least volume of fluid in your car, but also the least leaked and the least changed requiring disposal.

Also, I am not a Chemist. Of the sciences, it is definitely my weakest area, but I think that brake fluid is mostly alcohol and glycerine, niether of these are a threat to the environment.

This thread brings to mind a story on the news a year or two ago. It seems that the wife of some Hollywood loon was suffering severe, clinical depression due to worrying about global warming. I think it's time for the poor lady to start looking for some perspective.