3 Months ago I replaced the rear brake pads. About a month or so back I took care of a broken wheel bearing problem on the left rear. Maybe a week later this high pitched squealing noise started happening after 20 minutes of driving. Then after only 10 minutes. It was definitely my left rear brake and it seemed it was being pushed into the rotor while I was driving. It got to a point where it wouldn't squeal while driving but only when I was braking.

Checked it and the inside brake pad was almost completely worn down and the outside pad looked just like the rear right side pads did, lot of life left. They were only 3 months old. Fast forward 2 weeks to now.

I check the rear left again to fix the issue and the outside pad is now completely worn down, I mean all the way, more worn than the inside pads and the inside pad looks the same as it did a few weeks ago. The rear right pads still look perfectly fine, like they've been being put to work but plenty of life left.

What could be causing this issue? I'm going to bleed the brakes, possibly replace the brake line from the caliper. But my suspicion is in the caliper. It takes a lot of effort to push the piston in. More effort than the right side does.

 

That sir sounds like a stuck caliper piston to me. Only reason the pads would be worn down other then braking is the piston is stuck, which is then making the pad rub against the rotor during normal driving. That happened to my 97 Camry on my front left caliper.