Before you can know the truth, you have to look at the lies.
https://en.wikipedia.org/wiki/2009%E2%80%9311_Toyota_vehicle_recalls
From 2007 to now Toyota manufactured vehicles with electronic ignition, electronic brakes, electronic starting, electronic steering and electronic acceleration. In other words, there was no physical, mechanical failsafe for the control of the vehicles.
Time and time again they tried to blame it on something mechanical going on. Floor mats? No. A metal pin missing under the dash? No.
Here's the truth: they turned to a completely electronic control system and the damned things would wig out, the computer would lock all input from the user, and then max out the acceleration (pedal to the floor).
Grandma would be in a ride to hell in 6 seconds flat.
You might think "Oh, I'll just use the brakes!" Nope... They are electronic too.
"I'll just use the emergency brakes!" Nope... Discontinued.
"Oh! I'll take the key out of the ignition! Can't run without the key" Nope... You wanted the push-to-start, it is all electronic. Key in hand, the car is still in a death spiral.
"Well, shit... Here comes a wall..."
And then began the long string of out-of-court settlements paying off families to keep quiet about their loved ones dying in crashes exceeding 120 mph.
Now, what if that was just the trial runs for a new technology? What if they DeepState were simply introducing a new way to cut whistleblower's lives short? It would be quite easy to explain away as an accident. No proof, either, because the computer would most certainly be wrecked beyond function.
More cars than ever are running all-electronic systems. It is cheaper and more corruptible. You want your AI controlled driver? Well, be prepared to go to work every day knowing someone can press your ticket at the push of a button.
Dig deep. People have died. They continue to do so. Nothing new here. Just another distraction from the real news.
In every consumer purchasable production vehicle in the US, the brake pedal contains the master cylinders, so any sort of brake assist can only increase pressure, and perhaps dump a limited amount of pressure into an internal brake ECU reservoir for anti-lock-braking. The brake ECU is also independent from any engine ECU, with any sort of driver pressure assist based on a pressure transducer wired to the brake ECU. There could be some coordination over a comms bus for cruise control, but even as bad as individual firmwares might be, it would be incredibly unlikely for a glitched out engine ECU to cause a brake ECU to fail in a way that makes the brake pedal assist not work.
Also, if the brakes bring the vehicle to a complete stop with the engine at full throttle, they can hold it stopped for an arbitrary amount of time without dissipating any energy. There's no immediate need for the driver to jump out of the vehicle.
Huh, apparently later generation Priuses have two solenoids to completely block the master cylinder from the calipers during normal operation. They claim that those solenoids open if the brake ESC fails. That does seem a little sketchy, since firmware presumably must detect a "failure" to know to de-energize the solenoids, for a certain class of failures. Buggy firmware could therefore seemingly lead to loss of hydraulic brakes. By law, all cars in the US need to have redundant mechanical brakes. That doesn't necessarily mean that they can't be electrically controlled, but doing it all by-wire really drives up the stakes.
If they're smart, those solenoids would have an over-pressure bypass so that really mashing on the master cylinder would hydraulically connect the calipers again. That probably doesn't exist though. A brake failure coinciding with unexpected acceleration on a prius would therefore be rather suspicious. As far as I know, though, the bulk of claims about sudden acceleration in Toyota vehicles were for camrys, with normal redundant hydraulic braking systems.
Are you saying you've witnessed a prius go into service mode unexpectedly, or that you've seen a prius intentionally put into service mode and observed the loss of rear braking?
I'm guessing the rear brakes are disabled in service mode to be compatible with running front wheel drive on a dynamometer or something.