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.†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?
Re: Technical Question: How exactly do the power brakes work Heya, I've had my Prius for a month now. I can explain how the brake system works. You push the brake pedal and the car stops. Works every time. *ducking* Yoda
Look on page 292 of the wiring diagram. The ECU is called the SKID ecu, and the pump is part of the actuator assembly. Actuator assembly has the hydralic valves as well as the accumulator pump.
