How Prius Brake System Works

"I'd like to educate myself on how Prius brake system works."

Try this link for starters: Understanding Your Braking System (yes, it's on PriusChat. Note: an EXcellent way to find Prius-related info is to search thusly: '2012 Prius Brake System Design' [or whatever topic interests you], which is what I just did. In addition to many other links, you will almost invariably come up with a list of links from PriusChat in the search results).

Note: I'm NOT referring to searches in PriusChat, but searching on your preferred search engine.

 

Regeneration only happens on the front tires, as the car only drives the front tires.

Mostly, the car adds friction braking to regen, but in a panic stop, (when ABS comes on) it is all friction braking. (I often see that folks claim 75% of the friction braking is on the front tires, 25% on the back) Below 7 MPH, it is all Friction.

As if it did not already seem too complicated, when the big battery fills, the car adds engine braking to the front tires. (Confusingly, the harder you brake the louder and faster the engine sounds) You can invoke this anytime by using B mode, but MPG takes a hit except in the Prime. Use B down very long hills, 600 foot drop or more.

Just for completeness, there are cables to the rear brakes for the parking brake.

 

It's more complicated. It's been covered in other threads before. You could review this post for a start.



Although that image is from the 1st gen New Car Features manual, and some details change over the generations, it gives the idea.

The brake ECU and the power management control ECU are constantly exchanging information. The power management control ECU (horrible long name, I'm going to say p22l ECU) is more or less the orchestra conductor for the car. The brake ECU knows how hard you are pressing the pedal, and how fast the wheels are turning, and shares that information with the p22l ECU. The latter knows how much battery capacity is available to accept charge, and the temperature of the battery and inverter, and how much regen can safely be handled, and says to the brake ECU, "ok, I'll do this much, you do that much". And it recalculates and revises those instructions constantly from the time you put your foot on the pedal to the time you've come to a stop.

Accumulator and actuator are both names of real parts in the Prius brake system, which is endlessly confusing because the words are so similar and people are always mixing them up. Catching that shows you're really paying attention.

The accumulator is a tank for storing brake fluid under pressure. It has a factory charge of pressurized nitrogen gas and a barrier that separates it from the fluid. An electric pump forces fluid into the accumulator, and that stored, pressurized fluid is used to operate the brakes.

The actuator is a set of hydraulic passages, pressure sensors, and electrically-operated valves directing the fluid out to the wheel cylinders and back. It is how the brake ECU is able to control the brakes.

As you have a Gen 3, the accumulator and pump are together in the lower assembly shown here, and the actuator, master cylinder, booster, and ECU are all together in the upper one. Different generations have separated and combined them in different ways.



There is one accumulator supplying pressurized fluid for the whole system. But the actuator has separate valves, and is very much able to apply different amounts of pressure to the rear and front brakes. That is normally necessary, because of the physics generally requiring the front brakes to do more of the braking, and the proportion can vary depending on how the car is loaded.

When bleeding the rear brakes, you are seeing the effect of the accumulator and pump. Pressing the pedal causes the actuator to open the appropriate valves, and of course fluid will continue to flow as long as there is accumulator pressure. The electric pump will come on as needed to maintain the pressure. (You must be careful if you are letting fluid out at the bleeder; the pump can easily use all the fluid in the reservoir and then suck air in, which greatly complicates your bleeding job.)

Your description of bleeding the front brakes is incomplete. The actuator has two separate hydraulic circuits for the front brakes. One works exactly like the rears, and will continue to supply fluid as long as the pedal is pressed. The other is a fail-safe circuit that is only used if the electronic control of the actuator has failed. The fail-safe behaves like regular brakes because the fail-safe essentially is regular brakes.

People who bleed the front brakes incompletely, without going through the Techstream procedure for doing so, are only bleeding the fail-safe hydraulic circuit.

There should be an award for the most observant first post about braking, and you should receive it. These are all very sharp questions.

Yes, the reservoir level depends on how much fluid is hiding in the accumulator. On the reservoir, the level markings are positioned for when the system is "zeroed down": all of the accumulator pressure is relieved and all of the fluid is back in the reservoir. Any time you look at the reservoir when it is not "zeroed down", the level will look lower.

How can you "zero down" the system before checking your fluid level? Well, there are several ways.

There is a "zero down" command in Techstream. Send that, you hear a click-HIIISSSssssss... and it's zeroed down, you go look at your fluid level, super easy.

If the car has been parked and off for a good long time, the system is probably pretty close to zeroed down just through natural slow leakage, close enough that you can sneak a look at your fluid level. But you have to do that without opening the driver's door, which will awaken the brake ECU and pump the pressure back up.

If you remove the fuses for the brake pump, you can zero-down the system by pumping the brake pedal (in any Prius generation except Gen 2). You'll get around twenty or thirty strokes before the pedal suddenly feels quite hard. That's how you know the pressure is used up. Then go look at the level.

Edit: after posting quite possibly the keenest and most-observant set of Prius braking questions seen on PriusChat ever, you've deleted them ... ?

Edit edit: oh, I see, you moved it here. (I've cheated a bit, changing the post numbers in the quotes above to refer here, but the exact quoted words are still from the earlier very-similar post.)

 

If I’ve washed our 2010, and especially if it’s then sat a day or two, on the first subsequent drive I can HEAR rust scrubbing off the rotors, regardless if it’s light or moderate braking, speed and so forth. Bottom line: there seems to always be some amount of friction braking.