Technical Question: How exactly do the power brakes work?

As a new Prius owner, one of the first things I noticed is that the brake system master cylinder (on the firewall) does not have the typical (for other cars), “diaphragm vacuum boosterâ€. Which makes sense, the Prius is not a typical car, and as the Internal Combustion Engine (ICE) is not running all the time, there would not be a reliable source of vacuum. Thus the engineers must have designed the power brake system some other way.

Reading the Owner’s Manual, there is a hint on page 221, “The brake actuator uses brake fluid pressurized by the pump to power-assist the brakes.†This pump must be electrically powered, so pursuing this hint, I downloaded all of the 2005 Prius Wiring Diagrams from the Toyota Technical Information System (http://techinfo.toyota.com).

However, I cannot find anything related to a “brake pump†in the wiring diagrams.

I did find the “Brake Control Power Supply†(a black box mounted in the luggage area next to the 12 volt battery) which is described in the wiring diagram as: “Electric charge is stored in brake control power supply. If voltage of vehicle electricity is declined, electric charge is released to cover electric supply to the system.â€

So it’s quite clear that the “power†in the power-assisted brakes comes from an electric pump, and that pump has a back-up electrical power supply – but where and what and how does this pump system work?