The Magdraulic brake was an electromagnetic option brake made by The Empire Electric Brake Company and sold for Harleys and Indians in the late 1930's / early 1940's. This brake dosen't use a brake cable at all, only two 6 volt wires. I just came across one of these setups and I will be posting more pics of it in a little bit, but until then:
Here you go, the complete Magdraulic Brake sales brochure dated April 30 - 1940.
Thank you Totte for sending me the scans!