I just removed the pneumatic/hydraulic activator from a 1978 Sovereign
International that had had its disk brakes replaced with electric. Very interesting (and complex) mechanism. The service manual does quite a job explaining basic operation using vacuum from the tow vehicle and a standard electic controller line for the regulator solenoid. (anyone need parts?)
I think the electric brakes started to become most common in the mid sixties or so but that may not be a well founded impression.