Groove worn in drum brake pads

[renton]

I had my trucks front brakes overhauled and they did a inspection a few months ago and told me my back brakes were fine for awhile. I was changing tires and decided to check the back brakes and found that the passenger side brake pads had grooves worn in them. The driver side is starting to do the same thing but is just barely noticeable. I have no idea what would cause this, I've looked over it and can't see anything obvious and I've never come across the problem before. I noticed the axles have some end play but I can't see it being enough to cause this, I just changed my rear diff fluid and it looked clean and the gears seemed fine. I was planning on going up a mountain this weekend to go snowboarding so I hope this isn't some major issue.

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[adsm08]

One of two things is going on here.

1) You have an enormous rust lip (or at least you did) on the edge of that drum.

2) At some point something came apart in there, made that mark, and is now disintegrated.

 

[renton]

Ok thanks that makes sense, yes there is a big rust lip. I take it off-roading often so it makes sense maybe something got stuck in there.