Prius Brake Issues

When you gently brake you are mostly using regeneration via Motor/Generator 2, only from 7 MPH uses friction brakes.

More abrupt braking uses friction brakes at higher speeds.

One test of this is to brake in N, where it is all friction brakes.

 

The friction brakes are used very little on the Prius and are only engaged below about 7 mph. For this reason they are subject to rust buildup.

Something you can do to help this is to occasionally shift to Neutral while braking. Shifting to Neutral disengages the HV battery, disables regen braking and forces the friction brakes to operate. Do this a few times every now and then and this will help wipe any rust from the rotors and pads and help with the "sticking" issues.

There are also two TSB's that address both front and rear brake issues. Ask your Service Manager about these. There is a thread at Listing of 3rd Gen TSBs | PriusChat that includes the two brake TSB's.

 

We just had our hycam in to look at a brake issue. Hard stops or pressing the brakes half-way to the floor produced actuator or soft piston type sounds. Dealer confirmed it was coming from the sealed ABS actuator box (which contains small pistons that control the brake pressure) and that the loudness of the noise was not normal. The master tech told me he had seen instances with the 2nd gen Prius where bleeding the lines got rid of the noise (the hycam brake system being avariation of the Prius system). Since it is a sealed system with hydraulic fluids, he said they could never figure out why that should happen. We did not have them do anything, but they did top off the fluids. My wife is claiming that solved the problem, but I haven't tested it myself. Very odd. If the ABS box is actually going bad, the noixe should get worse and we should eventually get warning lights. $3,000 repair if that happens. I was shocked, to say the least.

Off topic, the dealer still got their pound of flesh by claiming we had a leaky water pump (at 5 yrs, 63,000 miles). I have my doubts about whether replacement was really needed, and about the cost ($500), but my wife didn't want to wait for a 2nd opinion, and the results can be sudden and severe if it really goes out while driving down the highway.

 

That's not true. There always engaged. Just regen assist is completely removed below 7 mph's.

 

Sounds to me like an ABS wheel sensor is slightly out of position and loses signal at very low speeds. I've have this problems on other vehicles (Teves systems, though) where cleaning the sensor seat and re-seating it was all that was required.

 

I have read in multiple places about "transitioning" to friction braking around 7 mph. There have been many reports of the brakes sticking when being applied.

Let me see if I can find some info on this.

 

Transitioning and never engaged are 2 different things. Of course there is a transition at 7 mph's.
The transition is regen is turned off.

But hydraulic functioning of the brakes is always on and working.
Its just assisted.

 

I think what you are hearing is the same thing I was hearing. The caliper pins and bushing have excessive clearances (slop). Mine was so bad it sounded like there was a component ready to fall off of the brake assembly when I hit a small bump. Also, when I back out of my driveway and apply the brakes I get a click from the rear calipers. This condition is supposedly remedied in aTBS by greasing the caliper pins. The pre 2012 models they replace the pins and bushings all together.

 

So can you grab the whole assy and pull it back and forth a little? That's crazy. What a factory FU. Everything has to be right in there or it will make a racket. And the type of lube on the caliper pins is critical. I have seen so many posts here with people saying they used synthetic grease on the pins.

This stuff here is the best. Its like candle wax. Every disc job I have ever done worked perfect and didn't make a sound when I was done. Pins and shim backing:

Permatex 24110 Ultra Disc Brake Caliper Lube 8 fl. oz. brush-top bottle

 

The original poster seems to be describing Electronic Brake Force Distribution (EBD) operation. EBD is like a partial ABS operation, in that it limits hydraulic pressure to one or more wheel cylinders (calipers), similar to what a proportioning valve would do. During a hard stop, the hydraulic pressure to the rear brakes would be limited, to balance brake force as the rear wheels unloaded. The brake actuator makes a rapid tic-tic-tic-tic noise when this control occurs. If lock up occurred, full ABS operation would follow, allowing decreasing, and then increasing brake pressure to individual wheel brakes.

The following is from the 2011 Prius New Car Features Manual:
"Electronic Brake Force Distribution (EBD)
The EBD control utilizes ABS, realizing proper brake force distribution between the front and rear wheels in accordance with the driving conditions. In addition, during braking while cornering, it also controls the brake forces of the right and left wheels, helping maintain vehicle behavior."

Posts #10 & #11 describe the clattering noise from the front calipers when you drive over sharp bumps, such as lane marker dots. The Prius brake calipers float quite a bit, in order to minimize brake drag. It appears the intent is to allow the caliper to freely release, when the brake pedal is released. When the brake pedal is depressed, the clearance is quickly taken up, to prepare for friction braking. The side effect is increased noise when driving over bumps or rough surfaces. The TSB to install bushings on the slide pins on 2010's and certain 2011's, and apply a specific grease to the slide pins and brake pad shims can help reduce the intensity of the noise for a while.
I was not aware of the Permatex caliper grease. It looks interesting, and I will have to look in to it.

Lastly, Toyota calls the basic brake operation as "Regenerative Brake Cooperative Control". Regenerative and friction braking are blended to meet the driver's intent. When the vehicle speed is too low, regen becomes ineffective, and only friction brakes stop the car.

 

Got it. Too much clearance creating to much room for the caliper to bang around. May be there solution for grabby rusted rotor/pad stickage.

But if you use to slippery of caliper lube it would be quite a racket. It would drive me batty as I hear all that stuff.
The permatex is excellent as it is not slippery at all. Its quite sticky but still a synthetic lubricant. And it stays where you put it no matter how hot the pads get. I bet they went to a stickier lube like this.

 

It would be the same influence as you would get by putting a gob of grease on the face of a hammer and striking it against a anvil. The grease cushions the blow with great effect.

 

Last Saturday, I was driving at a low speed (< 30) and heard a click sound when using the brakes. I have had the car since mid-December, and never noticed this sound before. Seemed quite loud. This past week, I dropped by the dealership and asked to have a master mechanic go for a ride with me so I could point out the noise.

I was told ---> "the noise being heard is coming from the brake actuater." I took another 2013 Prius out for a short drive around the dealerhsip and at first I could not hear the click, but then it became evident on that car, too, though faint.