Braking issue

That's really weird. I'd go to the dealer immediately, but there are some possibilities. But here are some questions to narrow down some possibilities. Was the car parked for an extended period (a month or more)? Could she tell where the grinding came from - front or rear? Was the pedal mushy? Did the pedal depress? Did she feel pulsing in the pedal?

Sometimes people mistake antilock brake pulsing with grinding. If she felt a pulsing pedal, it may be an antilock system issue. Also, the C has drum brakes in the rear. If something was not installed correctly at the factory OR somehow the drum brakes were not adjusted properly, they may somehow be binding.

These of course are wild guesses off the top of my brainpan without being there and not being able to look at the vehicle. I hope she gets the car in to the dealer soon. A major safety issue may be involved.

 

Well, it happened to me twice when I drove 25-30mph and slowly increased the brake(almost full bar to the left HSI). My gen III slipped forwarded for 2-4 feet then brake pads engaged! I'll let dealer check on 5k maintenance

 

I'm glad she is safe. It really sounds like something went drastically wrong. My best (wild) guess is the valve body of the ABS or related pressure system

 

Pedal to the floor usually indicates air or a leak in the system. Since the brakes came back right after, it likely rules out a leak. Have the dealership bleed the brakes, and be sure to grill them on actuating the ABS valves during the bleed process.

Good luck, and please let us know what they find. Glad your Mom is safe.

 

How much pressure did she put on the pedal? Did she apply her fully body weight, or at least 112 pounds (500 newtons, a metric measure)?

This pressure is a threshold within federal passenger car brake regulations. When power brake assists suffer just about any single failure mechanism short of a burst master cylinder, the brakes must still work in the old fashioned manual braking mode, i.e. no power assist. Regulations specify the maximum allowed stopping distance from a certain highway speed when a pedal pressure of 112 pounds is applied.

If she applied at least 112 pounds and the brakes still didn't work, then there is a design fault that demands an actual recall. If she applied only 25 pounds and quit because the pedal feel was completely different and unfamiliar, then she has become too habituated to power brakes and committed a driving error by not going into manual (i.e. no power assist) braking mode.

There have been sporadic reports of this happening in other Prii at parking lot speeds or warm startups, but not frequently enough to pin down. It happened twice in my 2010, startling me then clearing before much symptom information could be noticed. I wonder if it is a transient loss of power boost, reverting to pure manual braking mode without the stroke simulator to synthesize the regular pedal feel.

I haven't had the 2012 long enough to experience it yet, or to have any confidence that it might be gone. Three-four decades ago I was accustomed to all power assist suddenly vanishing at random times (due to engine stalls), but that muscle memory is fading.

This issue definitely needs more exploration.