Uneven Rear Brake Pad Wear

I have a 2010 Toyota Prius with ~100,000km on it. It is ex-lease and I bought it used from a dealership. So far it's been pretty good except for it seems to be a magnet for getting beat up from pot holes and dings and dents. It seems to attract them more than usual.

About six months ago the rear brakes started making a squealing noise at low speed. When you google that problem you will find that there was a TSB for it, but it didn't indicate it was a major issue and more of an annoyance so I just ignored it.

New TSB for rear brake noise | PriusChat

Last week while on a road trip I noticed a definite metal on metal grinding noise under heavy braking down a steep incline, but I am rarely in that type of driving situation so I just thought it was the rear brakes working hard for the first time in a while.

After the road trip I washed the car and noticed the rear wheel covers had a lot of rusted brake dust residue on it. Then I realized the rotors had some pretty deep scores and the rear pads were completely worn. So again I asked my favorite mechanic mr. google and found out that this is known problem with another TSB...
Rear brakes gone with severe uneven wear at 55,000 miles | PriusChat
After reading any post I could find on the uneven wear I couldn't really find any description of what causes this problem. This post is my experience with this problem and some discussion of the possible reasons.

I went to the local parts store and bought Wagner QC1423 pads, the local parts store rotor and a universal tool for rewinding the caliper piston.

I finally got to work yesterday on jacking up the rear of the prius and pulling the brakes apart.
This thread has a good diagram, which I think is from the manual on the frame extension in the rear of the car for a jack point. Jacking up the rear | PriusChat

I used this youtube video for instructions on how to take the brakes apart.

I also found this website very useful.
Toyota Prius Rear Disc Brake Pads Replacement Guide - Gen III 2010 To 2013 Model Years - Picture Illustrated Automotive Maintenance DIY Instructions

What I found was the outer pads were worn down to the metal and the inner pads were maybe 50% worn, but I had no idea why it was happening. I was expecting to get 200,000+km out of these brakes like the GenII seems to get.

I inspected the parking brake cable and it was very easy to disconnect and move back and forth. Everything looked fine there so I don't think it was causing a problem.

I then pulled the caliper slide pins out and cleaned and regreased with high temp brake grease. They were in great shape and there was no problem sliding.

Since I didn't find any problem I started putting everything back together. I unwound the caliper piston with a universal tool I bought from the parts store. When I put the clips and pads back in I noticed they were very difficult to position properly and move back and forth. I couldn't get the pads to slide next to the new rotor. Then I noticed what I think was the problem. There was a bunch of corrosion in the dado part of the caliper. I filed all the corrosion out and put the pads and calipers back together. I also put a bit of high temp brake grease in the dado to hopefully stop anymore corrosion. I put the calipers and pads all back together and everything seems to move nicely now.

I am not a mechanic so everything I did may be completely wrong and possibly dangerous. I am sure the proper thing to do would be to replace the caliper, but that was going to be a very pricey option.

I have been pondering this issue over night and I wonder if the TSBs that Toyota have release are just slightly undersized pads to that they can still move freely in the caliper after it is all corroded. It seems pretty clear that if the pads are really jammed in the caliper from corrosion then the application of the parking brake would force the pad on to the rotor and it would be able to release. The pad would then wear as you drive the car and the whole thing would repeat the next time you applied the parking brake. I did notice a marked improvement in acceleration after the brake job.

I am not clear on how the corrosion actually occurs either. It seems like the caliper might be made of aluminum and if that is the case it could be the two metals coming in contact or it could just be simple rust from salt and crap on the roads. My car does show signs of rust here and there on some bolts so was probably exposed to a decent amount of road salt.

Hopefully this write up of my experience helps people with the same problem. If you are in an area with high salt on the roads it might be worth getting your brakes checked.

I attached a bunch of pics that show what I was trying to describe.

 

Yes, I did do that, but only after I re-read the rear brake squeak TSB. For reference it is here: Listing of 3rd Gen TSBs | PriusChat

I had to take the driver's side apart to because I didn't catch that at first.

 

The pins were sliding smoothly with no problem, but I cleaned off the grease and put substantially more grease on.
I am positive what caused the caliber to jam and cause the pads to wear unevenly was the corrotion behind the metal clips.

Does anyone know if the calipers are made from aluminum?

I suspect the TSB for replacement pads reduce the width of the pads so the won't jam when the calipers corrode.

 

I think we are going to disagree on this point. I have been servicing all my cars for the last 24 years. I have replaced many brakes and I have never seen this kind of corrosion and I have never seen brakes pads get jammed up in the calipers.

My brakes were recently serviced and the brake fluid was flush with the brake booster recall. It is not a neglect issue.