2010 Toyota Matrix, Manual, Base model 1.8 L
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.
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?
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?