A few years ago there was a hydraulic accumulater setup that you tee'd into the front and rear brake lines.
The accumulater absorbed the pressure spike when you stabbed the brakes,and allowed the pressure to rise more evenly, preventing instant lock up.
The system was purely mechanical / hydraulic . ....air pressure adjustment to the accumulater controlled the sensitivity.
I know Jim Morrison had it on his S/G Opel GT a few years ago,but dont know the name of the manufacturer.
Found this on line , I think this is the one.
http://www.southwestspeed.com/?sec=view_part&id=1202