Rear brake piston hard to turn back in, uneven wear on pads

Question

2010 Toyota Matrix, Manual, Base model 1.8 L

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Some context

It was time to replace the rear brakes (noise, grinding). When I took everything apart, I found that the inner pads were extremely worn, at an angle - part closer to the hub was down to bare metal, and the outer pads had normal and even wear.

The slide pins were OK; I lubed them and replaced the rubber bushing and boots.

I suspected that the pistons might be seizing, so I took the calipers apart. No wear, no rust, nothing odd with the cylinder or the pistons. I changed the square cut seal and the boot, made sure the bleeders and banjo bolts were clear, then re-installed and bled the two brakes.

Took if for a test drive and the left rear was very hot (smoking actually). I had noticed that when I turned the piston back in, it took a very long time and required quite a bit of pressure, compared to the right side, which went in easily and in much fewer turns.

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The question

Given that the threads in the piston and threaded rod in the caliper looked fine, what could cause the piston to require so many turns and effort to go in? Does it point to a bad piston (internal mechanism), or a bad caliper, and how to explain it (more than just saying it's "bad")? How would you go about diagnosing the issue, before throwing parts at it?

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Unrelated?

Not sure if this is related to the issue, but also took a look at the front brakes. Found surprisingly little wear, despite the front having been done two years ago. Also found some pitted slide pins (not seized, but not great), but otherwise fine.

Is there any reason to suspect other parts of the braking system, or would the issue with the rear brake be enough to account for the uneven wear?

I've been experiencing a somewhat soft brake pedal, despite properly bleeding the system more than once. Would the rear caliper issue account for this?

Answer 1

In reading that you had pad wear at an angle the focus falls primarily to three locations.
The mounting bracket face being true (parallel) to the rotor.
One or both of the pin cylinder slides binding on travel either due to wear of lack of lubrication.
Or the piston and/or the caliper housing bore has worn to the point that the piston cants in the bore and is not retracting properly.

Based on your description the piston bore wear is likely as well as possible pin binding.

I see you have inspected and even replaced piston seals. Did you check the pistons slide in the caliper bore without any seals or rubber.
I test by putting a thin coat of brake fluid on the piston then slide it in and out by hand looking for any feeling of binding. There should be none.

As for your pitted slide pins, if pitted could be a possible catch point. In addition if pitted are they still true and evenly round.

Answer 2

After following @spicetraders' advice, I was able to conclude that there was nothing wrong with the rear calliper.

I disassembled it and tried the piston without the seals/boot, it went in fine in about 3.25 turns. With the seal, but no boot, however, it seemed to turn in place without threading in. I realized that there was a mechanism inside the piston that disengaged it if a torque was exceeded. You can see that when the piston bottoms out, you can still spin it freely.

So, reassembled everything, bled the brake, changed the front slide pins, and the brakes were back to a similar working condition as before, minus the horrible squealing.

Will keep the spongy feel for another question, as I can find nothing wrong with the calliper and breaks themselves.