Brake / ((!)) / ABS / VSC Strikes Again

This is a brief saga; hoping it helps someone or maybe responses will teach me more.

We had this happen twice to us this year - Brake / ((!)) / ABS / VSC lights all on constant and long beep upon starting our 2008 Prius. Looked like this:



The first time happened months after hearing a chattering pump sound (increasing in volume and length of time) from the engine compartment when opening the driver's door and starting the car, and later a chirping/barking noise when holding the brake peddle at a full stop. Once the warning lights appeared and stayed on constant, I read codes with my TechStream that told me the accumulator system failed. The brakes never actually felt different than what we were used to at that time, but knowing what was coming (from PriusChat), we of course replaced that part. Did the work myself after lots of research, cleared the codes, and drove for months with fantastic-feeling brakes, no dash warning lights, and no problems.

The second time, five months later, that same set of lights and the long beep returned and stayed on constant. What a sickening feeling... could that expensive accumulator assembly really have failed so soon?! This time, the brakes immediately felt different... much harder to come to a stop. While diagnosing a few days later, I carefully drove quiet streets and noticed an occasional grinding sound from the driver's-side wheel. Smelled "hot brakes" when I parked.

Checked codes with TechStream:
* C1246 - Master Cylinder Pressure Sensor
* C1256 - Accumulator Low Pressure
* C1341 - Hydraulic System (FR)
* C1364 - W/C Pressure Sensor
* C1365 - ACC Pressure Sensor
* C1391 - Abnormal Leak of ACC Press

I pulled both front tires to check the pads; expecting to see no pads, metal-on-metal (hoping desperately this was some designed fail mode of the system causing the dash lights)... but there was plenty of pad left on all four.

Then I saw something I've never witnessed before - the thin metal plate (anti-squeal plate, I think) delaminated from the steel backing of the pad and rotated outside of the calipers to grind itself into the hub. (Pictures below). This likely caused the wheel to studder a bit upon braking (I think I felt that a couple times)... maybe even triggering an ABS error resulting in the dash lighting up (like above). Maybe?









I got a warranty replacement on those pads, replaced them, reset the codes, and to my surprise the braking system returned to normal (excellent response, no dash warnings). It's been two weeks now and hundreds of miles.

What do you think? Would the ABS/VSC system respond this way, throwing errors and basically disabling the automatic system (very little assist in brakes) if something intermittently obstructed the wheel motion like this? I can't believe it, but I have no other explanation at this point.

 

Right, my thoughts exactly, but then my dad (a truck driver) reminded me that he'd had that same thing happen to him on his rig and it locked up the wheel long enough that the tire blew, so maybe there's something related here. Still hard for me to believe though.

Yes, brand new, $1,200 part direct from Toyota. Had to call around the country to find one since there was a supply chain issue and most dealers were way back ordered. I wasn't going to mess around with any used parts; critical system on a Gen 2- any of the best used parts would be old and likely well-used too.

 

Thanks for your thoughts; reflects my own too. I'm still leery of fully trusting it... but now we've been through even a serious rain/wind storm for a few days with no degradation in performance. Also, thanks for bringing up the C1341... does that (FR) mean "Front Right?" That's what I'd figured, but there were no visible issues on the front passenger's side.

Also, I didn't mention it, but I pulled both speed sensors (ABS sensors?) on the front to check them for debris. Both were cleaner than I expected. I wiped them and did not see anything unusual in their access ports.

 

It's been a few weeks and apx. 1,000 miles of normal city+highway since my last update; still no problems.

re: connector - Not likely; I should have had another incident from vibration by now, and I'm confident I made sure all the connectors clicked into place while I reinstalled everything.

re: boost pump - After car sitting for hours, touching the handle (to unlock) and opening the door... totally silent. (This was definitely NOT the case when our old accumulator system was failing; the boost pump would start running as soon as we unlocked the door and would continue for 1 min or so). With car off, pressing on brake the brake activated the boost pump for about 4 seconds, then stopped silent. Turned car on to Ready mode; still silent. Release brake and wait, still silent. This is exactly as it should be with a properly-working brake accumulator system.