So... if you'll remember, I'd hit this roadblock in my quest to have power brakes with the big ol' Four-Point-Six-Liter-Chevy-eater under the hood...Short answer: Because the 4th-gen Prius is brake-by-wire.
Long answer: Because the 4th-gen Prius is brake-by-wire, meaning that the master cylinder isn't so much a master cylinder, as it's a hydraulic multi-purpose device with the primary function being to give the driver of a Prius the feel of normal hydraulic brakes. The module on the side of it is there to use inputs from various sensors, including the yaw rate sensor in the steering column, the brake pedal position sensor, various pressure sensors, the wheel speed sensors and so forth and so on to decide how much of and which type of braking to apply. The "which type of" braking comes into play because the majority of the time, the Prius isn't using it's hydraulic system and brake pads to stop the car, it's using the electric motor for regenerative braking, which not only slows the car down, it also recharges the high voltage battery. While stepping on the pedal connected to that master cylinder does cause brake fluid to come out through two of the three ports on it, it's only designed to do so under certain circumstances, in others, it bypasses internally and sends the fluid to a "stroke simulator". In addition to all of this complication, there's also the fact that while the assembly I got DOES include the brake booster, it's not the WHOLE brake booster. If you look up at the hydraulic diagram, you'll see the mention of a "brake booster pump assembly". That's not internal to the assembly I ordered and received, it's halfway across the engine compartment. That guy gets fed brake fluid via gravity via the nipple on the side of the master cylinder reservoir through a hose, and then sends pressurized fluid back to the booster's inlet port. Without that, no boost.
On top of ALL of that mess, when the Prius master cylinder's control module goes into failsafe mode, it does so by reverting to behaving as a manual brake master cylinder, meaning it won't energize any of it's various internal components to do a damned thing to help, and even after consulting a Toyota master technician I've known ten years and putting our heads together studying the various wiring diagrams, hydrulic diagrams, and parts catalog pages, neither of us is convinced it could or should be tried. The one thing we both are sure could be done is using a relay that's triggered by the stoplamp switch to turn the accumulator pump on and off, but we can't be sure the pressurized fluid would actually go where we want within the master cylinder assembly.
The electronics actually run it as two engines, it has a separate DME (PCM) for each bank, and they're linked together through a single harness.Those engines always amazed me with the turbos in the valley. Pretty smart set up...but complicated to split the intake to opposite sides. That thing is almost like two engines smashed together.