Uneven brake pad wear, replaced twice

Even for someone with lots of disc brake experience, Prius (rear especially) brakes present a few unusual critical tricky bits.

Pretty sure the estimable Mendel Leisk will be along shortly to put you in the picture...

 

Is it the rear outer pad that is wearing unevenly? To me, it looks like the caliper is pressing on the top of the outer brake pad causing the wear to slant. This happened to me about 2-3 times now. It seems to always happen after the winter months.

I have replaced my rear brake pads three times now (once with the rotors). I use wagner thermoquiet brake pads + centric rotors.

I replaced the caliper pins. That didn't work. Well, one original pin was rusted and later sanded.
I replaced the caliper boots and bushings. This didn't work quite work.
I sanded/filed down the outer brake pad ears. Not sure if this worked yet as I did this last spring.
I bought 3M slicone lube to replace my prestone caliper lube. Not sure as I did this 2 years ago.
I replaced one of the rear OEM calipers 3 months ago with a Raybestos Element3 caliper to see if it works better. I originally wanted to replace the caliper mounting bracket, but it costs as much as entire aftermarket caliper assembly.

I will have an opportunity to check on my brakes the next time I swap tires. My next possible update is when winter is over.

And yes, you have to make sure the piston "X" face clears the brake pad backing plate pin. This clearly wasn't my problem as the inner brake pad (which the piston is in contact with) is always wearing evenly. I always had uneven wear with the outer pad.

I still have my old pads. I can also take pictures, but I don't want to get my hands dirty.

 

Gotta walk the dog first, but rest assured, there is an explanation. In the interim, some info attached. Look in particular for this:



It's imperative to reassemble with the piston oriented thus, and have it remain thus.

 

To this day I'm a little confused by the dynamics. Here's a pic of the pad backs, with these stubby pins flagged:



The caliper piston has to be oriented like an X (per previous pic I posted), so that those pins are between the raised spokes, the pattern on the piston face. Then the pads get even bearing.

Applying the parking brake, I believe that piston will attempt to rotate. What stops it, hopefully, is that pin, in between the spokes.

What's still puzzling to me: if the piston's managed to rotate, and the spoke has ridden up on the pin, why is the pad wear bevelled towards the edge opposite, without the pin (the edge with the "ears")?

FWIW, I had the same issue, and similar wear. Also, on the inside face of the rotor, the pads seemed to be making next to no contact, as evidenced by the rusty zone:



My pad wear:



It's a little subtle, but I believe the wear zone is on thumb side.

Anyway:

Your rotors likely have a rusty zone on the inside face. I would removed them (mark a wheel stud and the rotor hub to reinstall in same oritentation), and cleaned them up with steel wool or wire brush. They will still likely sound terrible after, for the first little while, but it helps. M8x1.25 x 1" long bolts screwed into threaded holes on rotor hub will help to bust them loose, if needed.

Buy another set of pads, Toyota preferably. Prep them with the shims, thin coat of anti-seize on all faying surfaces, between pad back and shim, shim and caliper.

Get the caliper piston orientation correct (per pic in previous post).

Install everything. Pump the brake pedal multiple times. This helps to seat the pads solidly, and also avoids the car detecting excess brake pedal travel, throwing a code.

Then reconnect 12 volt neg cable (it should be disconnected at the start, before any disassembly, to avoid possible activation of the brake pumps.)

Do not apply parking brake.

Lower the car. Take a short test drive. The brakes will likely make noise with every application, due to residual rust on rotor. On return, apply/release parking brake several times.

Raise rear of car. Spin wheels. They should easily spin 2~3 revs with a good push.

If they do, you should be ok. Brakes will continue to be noisy for the next day or two, but should clear up.

 

Did you clean up ALL the matting surfaces where the pad metal touches the caliper?
And did you use the brake paste/lube in those spots? Did you use the original plates from the
original pads, or new ones, on the replacement pads?
From what I understand you said the OUTER pad is wearing, NOT the inner?

 

I doubt it's the calipers.
You should scan youtube and watch videos of how to do the rear pad replacement.
It could be possible he has the pads switched. Inner pad on the outside, outer pad on the inside.
Do you have the correct pads? If you get Toyota pads from Toyota, you should be good.

But you may have to let Toyota do the job. And ask them to check it carefully and see what the problem is.

 

Did the guy who did your previous brake jobs know about the piston orientation?

 

Hard to tell without seeing the rotors....

 

It caught me flat-footed, the first time I did a rear brake inspection. 6 months later I noticed scoring on the outside of face of the rotors. I recalled the wheels seemed draggy when I'd buttoned everything up, thought it would smooth out with time.

Nope: jacked up the rear found it was still hard to turn the wheels. Took everything apart, took some pics (posted above). Read up on the piston orientation requirement, purchased new pads, took everything apart again, took off the rotors and steel wooled the worst of the rust off the inside face.

Then put everything together with new pads, paying VERY close attention to the piston orientation, pumped the brake pedal, firmed everything up, and didn't touch the parking brake. Tested the wheel spin, seemed pretty good. Took it for a test drive, easy on the brakes: lordy you knew when the brakes were being applied, very noisy. Due to that rusty, untouched zone on the inside face of the rotor.

Got back, depressed/released the parking brake, raised the rear, and confirmed the wheels continued to spin pretty good.

A few days later the noise from the brakes was hard to hear. A month later perfectly quiet, and the scoring I'd first noticed on the front face of rotors was gone as well.

 

I'd never heard of this before, but then all our previous cars had rear drum brakes. It's due to the integrated parking brake, and the particular design. There is another approach that utilizes a mini-drum brake, built into the rear rotor; the Prius v uses this approach. our daughter's Honda Pilot had the mini-drum too.

Yeah, if you've got a finicky, somewhat unusual design, that can be messed up if you're not on your toes, maybe that's not a good design...

 

Now you have the proper instructions. Have your friend do it a third time, this time with the directions kindly shared by Mendel. (My humble opinion)....

moto g(7) power ?

 

One thing to be aware of: heat can be the enemy of the rubber parts. The rubber seal inside the caliper plays an important role in retracting the piston when you are done braking.

There is a possibility that the rubber was originally fine, and there wasn't any problem except the pad installation, but with the pads constantly dragging, the brake may have gotten hot enough for long enough that now there is a cooked rubber seal added into the picture. If that has happened, even with the pads installed right, the brakes may continue to drag.

It's possible with a dial indicator to check that the caliper pistons have good, positive retraction (I've measured around 0.3 mm for a good Prius front caliper, but I haven't measured a rear).

If the retraction is shot, the rubber kit for caliper rebuilding is not much money (though, unlike a simple pad replacement, you have to not mind brake fluid on your fingers, and be able to bleed the system when done).