VSC, ABS, (!) and emergency brake light on, replaced 12v battery but still brake issues

So Toyota degrades the safety of the car on purpose ? I find that hard to believe.

 

I would not say that Toyota degrades the car's safety on purpose. Rather, I would say that when the skid control ECU and the hybrid vehicle ECU determine that there is a braking system failure, that several warning lights associated with the skid control ECU will turn on and the system operation will be impaired.

The Prius braking system really is electronic, "by wire". If there is a problem with the related electronics, you cannot expect that the system will behave like nothing is wrong. This impaired behavior provides a strong motivation for the driver to take immediate repair action, instead of thinking that the problem can be ignored.

The following text is from the factory repair manual and a careful reading will uncover a few errors as the text was translated from Japanese to English:
"OPERATION DESCRIPTION

ECB (Electronically Controlled Brake ):

The skid control ECU receives signals from the pedal stroke sensor, master cylinder sensor and wheel cylinder pressure sensor. Based on these signals, the skid control ECU calculates necessary braking force for each wheel. The necessary hydraulic pressure braking force signal is sent to the Hybrid Control ECU via CAN communication. The skid control ECU receives a braking force (regenerative braking force) signal from the Hybrid Control motor via CAN communication. The ECU calculates the necessary hydraulic pressure braking force based on the necessary braking force and regenerative braking force.

Necessary hydraulic pressure is supplied to each wheel by adjusting the brake accumulator (hydraulic pressure source) pressure with each solenoid valve.

If there is a problem with braking function, rest normally operating parts will maintain brake control as a failsafe."

 

If this reoccurs, I would personally go to one of the two excellent independent shops there with a used part, and have them install it, if that is not what you want to DIY.

BTW, that would be "voila", a French word: Voila - Definition and More from the Free Merriam-Webster Dictionary

 

Very fair point, and thanks for posting the description of how the system works.

But now I have to say I cannot imagine what the failsafe mode would be if the brake pedal position sensor is thought to be malfunctioning.

 

"I cannot imagine what the failsafe mode would be if the brake pedal position sensor is thought to be malfunctioning."
The OP provided a fair description: "...the brakes got really squishy feeling. They basically, took longer to stop and when the car did start slowing down, it was like I was pumping the brakes, even though I wasn't."
That is similar to the experience my dad had when the 2007 had a similar problem. I posted about his experience several years ago since at that time, it was highly unusual to have any problems with the electronic braking system. See my posts #45, #48 and #54 here:
BRAKE FAILURES ON 04+ PRIUS | Page 3 | PriusChat
There are two more similar stories related, later in the string.

For another example, see:
Brake problems, Stroke Sensor, and Dealer Service | PriusChat

 

Yes, I read the description but I cannot translate it into symbolic logic at the level of the brake brain (BB):

BB: I do not know how much the brake pedal is depressed.
BB: Maybe I do not know if the brake pedal is really being pressed at all.
BB: What should I do ?

BB: Brake a little ? Brake a lot ? Alternate and rely on directional input changes from the brake pedal sensor ??

I'm not an engineer, but it sounds like a really tough problem to me.

 

The brake pedal stroke sensor is composed of two parts, probably variable resistors or the functional equivalent that will translate brake pedal movement to a variable voltage.

The skid control ECU provides an input voltage ranging from 3.6V - 4.95V. When the brake pedal is released, sensor 1 is supposed to produce a voltage of 0.7V - 1.3V while sensor 2 is supposed to produce a voltage of 3.7V - 4.3V.

The zero point calibration establishes for the skid control ECU what voltage to expect from each sensor. The problem occurs when the two sensors produce voltages which call for differing braking action. Then the skid control ECU is left clueless as to what to do.

The end result is poor braking which will scare any driver who is lucky enough to experience this issue.

 

My 2006 Prius with approximately 106,000 seems to be doing this too. I was running errands and came upon a red light. I stepped on the brake peddle and the brakes felt like they 'skipped' - for lack of a better way of explaining. At the very moment the brakes 'skipped', the ABS, VSC, red Brake indicator and this



light came on the dash.

Any good independent hybrid repair shops near Kent, WA?

 

I have no specific experiance with this, however I would say there is a reason I call dealerships, stealerships. I think your low batt caused an error to occur and by clearing the code you will likely not have any more issues with this. Hope that is the case

 

I just brought in my 2005 Prius with these symptoms to an independent shop. I was told that the skid control computer needed to be calibrated, which cost me $160 (including diagnostics and labor). I was further told that if the issue returns, the computer will have to be replaced for $2100 (including parts and labor).

These are the technician's notes:

 

An alternative is to have a used skid control ECU installed. You can probably buy one on eBay for a low-three digit amount. The hard part is installation because most of the dashboard and the steering column have to be removed for access.