The system is basically an electrically driven hydraulic pump with an oil accumulator and a hydro-boost power unit attached to a standard master cylinder. Audi 5000 had a hydro-boost system but used an engine driven hydraulic pump. I believe some GM trucks also had/have a similar system. Not a bad idea but a vacuum pump is pretty cheap and uses the existing power assist unit. The Audi system was always leaking and uses a special mineral oil that costs about $22 a liter and the components had a high failure rate compared to a vacuum assist system. As for a vacuum pump system, I always thought one could use a vacuum pump from a Ford or GM diesel truck and drive it off the EV drive motor. A large enough vacuum reservoir would provide enough assist when the motor isn't running. When enough vacuum is created the pumping diaphram would retact and there wouldn't be a power drain on the motor. Mercedes-Benz was working on an electric brake system similar to trailer brakes. No hydraulic fluid at all. Pure electric calipers. Sounds interesting but perhaps not fail safe? Probably the same thing was said about hydraulic brakes when they came out when most cars had mechanical rod brakes.
