Overly sensitive breaking when in reverse

Rusty rotors. Need to perform derustification of rotors.

 

You mean braking and not breaking ! Expanding on edthefox5's comments, one way to clean the brake rotors is while doing normal stopping, shift to Neutral. This disengages the HV battery, disables the regen braking and forces the friction brakes to work. This action will help wipe the rotors clean. Do this a couple of times and your rotors should be cleaned.

 

I'm not convinced that this is related to rust. My 2010 Prius rotors were always much cleaner than my Subaru rotors, yet still displayed this anomaly a few times. The Subaru's pads actually do stick to the rotors, but 'break' off with a slight snap, leaving a rusty outline of the pad on the rotor, but never have this reverse sensitivity. If anything, its rust acts as a weak lubricant, requiring slightly increased pedal force until the rust is removed.

I still have a suspicion that there might be unidentified quirks remaining in the touch-and-feel layer of the Prius braking system: less-than-seamless transitions from regen to friction mode, rare transient dropouts of the power assist or stroke simulator or something similar, and this infrequent reverse sensitivity right after startup.

Nothing has caused me any concern about the basic underlying hydraulic braking system. It seems to work correctly. But while federal safety regulations allow power assist brakes to revert to old-fashioned manual brakes in case of power assist failure, many drivers have now lost (or never developed) the manual braking reflex of putting their full body weight into the brake pedal. So when the pedal feel changes, there are likely to be some operator errors.

The 2012 hasn't displayed any similar behavior yet, but I haven't had it long enough for that to mean anything.

 

amm1978,

I'm not sure if this applies to other generations but have you ever heard of a whirring noise when you get close to your Prius? Almost like it's purring to see you?

Well that sound is actually your regenerative brakes powering up so that they can be used right away. I've read that if you wait around for too long (say you're fiddling in your car, plugging in your phone, maybe plugging in a destination into your Nav, etc etc) the electronic braking mechanism may lose the power it primed earlier and you're stuck with pure friction braking which can be very very touchy. I've never experienced this myself so I can't speak from a first hand experience sort of situation but this is what I gather you may be experiencing.

 

This has happened to my 2010 Prius IV many times over the last 3½ years. It seems to happen when the car has been parked for a short period of time and then restarted and put in reverse. The brakes are fine when you park and suddenly then are ultra sensitive when in reverse. The slightest touch to the pedal and it shutters the car to a hard stop. I have found that if I bring the car to a full stop, turn it off and restart it it fixes the problem. There have been complaints from many 2010 Prius owners about the braking, but Toyota claims there isn't any problem. I don't agree.

 

If the brake pedal is sensitive, and you can safely, take your foot completely off the brake pedal for a split second. When you reapply pressure, the pedal will feel normal. Remember that the brake pedal operates by wire, not by hydraulic feedback...

That, or put some mobile oil on the breaks. (I couldn't resist using two of my favorite misused homophones).

 

I've had exactly the symptoms the OP describes: super touchy brakes, just at start-up, just once in a long while, only in reverse, and the behaviour stops as soon as you get out of reverse and get going.

The first time it happened I was backing up in a parallel parking spot, and could have sworn I'd hit the guy behind me: I touched the brakes and came to a sudden, jolting stop.

I don't think it's rusting brake brakes, because of the various descriptors in my first paragraph. Seems like a software and/or mechanical bug. Very infrequent occurence.

 

I also haven't thought it was related to rust -- the only thing that has worked for me when the brakes are touchy in reverse gear is to stop the car when the brakes grab, shift into Park, remove my foot from the brake pedal, then press the brake pedal, place in Reverse gear and resume backing with no further problem (until the next infrequent time this happens). I've tried placing the car in neutral when moving and using the brake to stop and cleaning the rotors like that hasn't stopped the brakes from grabbing.

 

I think I might have actually made this happen one time. Before I powered up, I held the brake down very hard and pressed the power. Put in reverse, let off the brake, backed out of garage, and applied brake. The pedal because very hard and very touchy. That was just a few months ago and I have been meaning to try the experiment again but forget to do so... mental note, try experiment again.

 

Same symptoms with my 2010 Two. I aways thought that it was me applying too much pressure on the brake pedal.

- Spindifferent

 

I have a (March) 2010 trim level III/3.

If I get in my parked car, step on the brake pedal, press the start button, keep my foot on the pedal, switch into reverse, and then ease off on the brakes to start backing up, I get the "sensitive brakes" problem. If, instead, I step on the brake pedal, press the start button, release the brake pedal, step on it again, switch into reverse, and then ease off on the brakes to start backing up, I don't get the "sensitive brakes" problem.

I haven't experimented with this thoroughly enough to add "never" and "always" to the above sentences, but as far as I can tell that seems to be the pattern with my car.