I took off my brakes pads to clean. I was getting occasional squeaking so I figured there was brake dust that needed cleaning. When I removed the pads, I noticed that on driver's side, one brake pad was worn out more than the other one; probably 40% to 60%, in other words, wear was not even on both sides. What could have caused that?

Assuming that the pads are of identical type and the same age, the only thing I can think of that would cause uneven wear is if you have slight brake drag due to a caliper not fully retracting.

Seized or sticky guide slide pins are the most common cause, making the inboard pads wear faster than the outboard pads. There are other causes for uneven wear but the pins are the most common cause if it's the inboard pad that is wearing faster. The following two video clips show you how to fix the problem. The first clip focuses only on the guide pins. The second clip covers other causes as well as the pins.
https://www.youtube.com/watch?v=Gdc3PZ0gFvA
https://www.youtube.com/watch?v=Qhjk2d0il_w

^ This is correct.

Clean, polish, and lubricate the slide pins.

As long as it's a relatively minor difference, I'd say it's normal. To me it's a given on any car that brake pads have uneven wear. Not 90-10 or anything, but just some difference. Not disagreeing with the above answers, but from your description I wouldn't sweat it.

On my wife's 2001 QX4, I find that I need to clean/lube the guide pins every 10k miles or so, to keep them moving freely.

I recall when I changed out pads and rotors over a year ago, one guide pin was a PITA. On the passenger side where pads wore out evenly guide pins were gliding nice and smoothly. I will take it apart and redo the lube job from scratch. Great feedback.

Really at that age, you should replace the calipers, pins, pads, metal shims, rotors, and the flexible brake lines. Also a completete fluid replacement.

Thommykent wrote: ↑

Fri Aug 14, 2020 9:18 pm

Really at that age, you should replace the calipers, pins, pads, metal shims, rotors, and the flexible brake lines.

I think that's way overkill. Maybe on a British sports car from the 60s, but on a Japanese car built this century I think it's a bit much. Replace calipers if they're not leaking? Flex lines can swell up after only 20 years or so, but it's rare. Now rotors and pads if needed with todays pad materials I might agree with you, might, but the rest I think is a waste of time and money. Nothing wrong with doing it, but it shouldn't be needed except in rare circumstances.

I changed the culprit guide pin and it is perfectly gliding now. I agree it would be overkill to change out parts that are still in good condition.