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I can tell when the friction brakes kick in but that's not the problem I've seen.
My friction brakes grab and release 2-3 times while slowing gently from 6 mph down to a stop. So far I've ignore it on the assumption that my brake pads are not broken in yet since I only have 1200 miles on it so far.
 

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I agree, I notice an uneven braking effect at very low speed as well. It doesn't feel exactly bad, but it does feel odd. I'd sure like to hear that this is OK from an expert or long-time owner, though...
 

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kk6yb said:
I can tell when the friction brakes kick in but that's not the problem I've seen.
My friction brakes grab and release 2-3 times while slowing gently from 6 mph down to a stop. So far I've ignore it on the assumption that my brake pads are not broken in yet since I only have 1200 miles on it so far.
This doesn't sound normal to me. My brakes don't "grab" or feel uneven (other than slight differences that can be expected when switching between regen and friction). I don't remember any different behaviour when the car was new either.

As long as it's stopping the car OK, you could wait to see if it evens out after a couple thousand miles, but I'd bring it into the dealer.


DGStan
 

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How do the brakes work? Does software control when it's regen and when it's friction? Or do the calipers contact after a certain amount of brake pedal stroke? What if the engine is off (e.g. being towed, or rolling down a hill)? Are they power brakes? I haven't yet found good info on how the brakes work.
 

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OK. There is a master cylinder, but normally its lines are cut off by valves, though one side is fed to a stroke simulator so that you get the feel of normal brakes.
The skid ECU reads the speed and position of the brake pedal to determine appropriate deceleration rate desired. Also there is a pressure sensor in the master cylinder lines, but I am not sure if those are used to detect desired braking, or only to check brake system health.

Once the skid ECU determines how much brake force is desired, it asks the HV ECU how much it can contribute in regen braking. HV responds in what it can do. If regen can handle the whole brake job, then nothing more occurs.
If regen is short on brake force for what is desired, the Skid ECU uses the friction brakes to cover the difference.
Panic stops, ABS, VSC or traction activation reverts strictly to friction brakes.

There is a brake pump that creates brake fluid pressure in a reservoir called the accumulator, much the same way air brakes do. Valves open and close linearly to apply or release correct pressure to apply the individual wheel cylinders.
Because the brake system is drive by wire, and there is great fluid pressure that is immediately available by a pump, the brake system can seem touchy, since you don't really have any feel of the actual brake pressure to your foot.

For safety's sake, there is backup electrical power for the brakes using supercapacitors located in the sub-hatch. Also, should the computer lose control of the hydraulics, such as the accumulator pressure loss, then valves would be set to allow the master cylinder to have direct control of the front brake calipers. I have posted a diagram of the brake hydraulic circuit almost a year ago.
 

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It sure sounds complex and heavily reliant on software. Will have to study the diagram. Surely the engineers went through this carefully in their failure modes and effects analysis. On the aircraft I work on, we have fly by wire systems that would scare the bleeps out of a non technical person, but the airplanes are reliable.
 

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Even though the system is complex, I would say that there is sufficient data that indicates that it is safe. After all, we did hear about the car stalling. Don't you think we would have also heard if brakes started failing? It is true that planes have been heavy users of fly by wire technology for some time yet we still deem aircraft safe. I'm sure that any system that is critically important (like brakes) has some redundancy built in so that the multiple failures it would take to disable the system are statistically extremely unlikely. Rememember, mechanical components (hoses, master cylinders, etc.) can fail also.
 

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Grabbing brakes

Just to kind of get back to the original question... yes, I have noticed "grabby" brakes at speeds less than 6-7 mph, but mostly only during wet weather. I've just gotten used to it after 40,000 miles.
 

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And then again, back to the brakes, Toyota gives you two idiot light warnings on the Prius. Both related to hydraulic pressure loss either by a leak in the system or hydraulic pump failure or accumulator failure.
Each of the red warnings admonish you to pull over to the side of the road and call Toyota for a tow to a dealership. The odds of having dual failures with the system (pump and accumulator) at the same time are about the same as those of you winning the lottery and it is a good system as long as you heed the warning lights.
 

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Hobbit said:
http://techno-fandom.org/~hobbit/cars/brakerust.jpg
.
_H*
That picture explains a lot considering that it would take some significant braking force to remove that rust with the friction brakes. You could generate friction braking by finding a nice place clear of people and cars and doing some quick acceleration followed by some very heavy braking.

Considering that the Prius uses regen almost exclusively in normal braking until 7mph when friction kicks in, I can see how rust on the surface of the rotor might build up and cause some strange braking sensations.
 

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Actually it takes surprisingly little non-regen braking
to clean that right off. But you have to make sure regen
is disabled, by shifting to neutral before starting the decel.
You want to brake reasonably firmly but not anywhere
near panic-stop level, but more importantly SMOOTHLY
so that brake torque is nice and even through the entire
rotation. You'll probably hear the rust noise
substantially stop during the first run of this.
.
_H*
 
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