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Excessive rear brake pad wear - wear pattern on piston - caliper rebuild?

2311 Views 15 Replies 9 Participants Last post by  strom_r0m
Hello internet,

yesterday I noticed my rear brake pads being completely worn out at 44000 km. Last time I checked at 35000 km and they were fine. The brakes were dragging a bit at that point so I cleaned the caliper and pistons and did all the sliding pin maintenance. The brakes were still slightly dragging afterwards but the rotor didn't feel hot to the touch so I left it at that. I can't tell if that has changed in the meantime.

I don't use my rear brake much and don't rest my foot on it. I recently filed down the rear brake push rod at 42000 km to lower the brake pedal (see this thread: Rear brake pedal adjustment bottoming out after foot peg...) and tested the brake pedal forcefully on several occasions.

Today I took the caliper apart and noticed wear on the piston on one side only.

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pads good at 35000 km
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Top Piston seal bit dirty but looks ok, bottom seal seems fine too
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Oooops, I wish I hadn't pushed it back in.....
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brake fluid from the calliper quite dark, last flush 8 month ago
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Only in the photo the damage was obvious to me, just looking at it I could easily have mistaken it for some stubborn crud
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opposite side looks better, just some very hard to remove crud?

Couple questions:

Did the piston get stuck and could only move back on one side which caused the damage? I'm trying to understand the mechanics of it...

Could my modification to the push rod have anything to do with it?

How likely is the piston responsible for my woes?

Could I be ok using the old piston and new pads and just see if the rotor gets hot?

I'm going on a 6000km trip to Greece soon. I'd be happy if anyone could share their experience with brake and piston wear...

Thanks!

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Discussion Starter · #6 · (Edited)
Thanks all, appreciate all your feedback. @richlandrick the housing looks fine, i don't see any place it could have rubbed against. I presume dust gets in the seal, crud builds up, the coating gets ground away by the particles in the seal and then corrosion sets in and thats the damage you can see now.

I ordered a used caliper on ebay for 40 bucks including shipping (compared to 55 Euro for Piston and seals). I'm already a bit weary of the condition it might be in but it goes straight back if there is any pitting on the piston too. I'll update when it's here.
 

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Discussion Starter · #8 ·
Brake lines in good shape. Have seen on vehicles, lines break down internally and hold residual pressure.
15 year old stock lines, nothing unusal to report when I last inspected them, dunno what that says about the inside...

I'm planning on replacing them at some point but without any apparent damage I wasn't in a rush to do so. I haven't read many posts here regarding problems with old brake lines.
 

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Discussion Starter · #11 · (Edited)
When I experienced brake dragging, I found my slide pin had developed a groove. OP's is visibly dry and corroded. It doesn't take much grease, you could apply it with a toothpick.

It might seem counterintuitive, but you should be using the rear brake more so that it doesn't seize up.

In 150k miles, the only rear brake work I've needed to do is change the pads, grease the contact areas, and replace the disk (after it dragged causing excessive wear).
Interestingly, upon cleaning and further inspection there is actually some marginal pitting on the slide pin that I hadn't noticed. Again, magnified in the photo though.

Might have contributed to some sort of seizing up of the whole slidepin-brakepad-piston arrangement. Not sure ...turning the pin 180° when reassembling sounds like a good idea anyway.

I did grease the slide pin at ~35000 km when I cleaned and lubed all the parts following the advice here on the forum. I had mixed feelings about lubing that part though because it does get filthy in no time and the lube exacerbates it and I didn't appeal to me cleaning and lubing it all the time.

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On a sidenote: I had similar pitting on both front brakes on the marked pin (not my brake in the photo). I also read descriptions of that issue here on the forum. No issues with the front brakes so far though.
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I wonder if this is unavoidable with this type of brake (combined with an unwillingness to clean constantly) or if the brakes on the wee are extraordinarily crappy.
 

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Discussion Starter · #13 ·
That's the condition of the piston in the used brake caliper that I bought...I'm not thrilled....

I guess all pistons more or less look like this though...is it worth a try or will i send it back?
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Discussion Starter · #16 · (Edited)
Can you catch your nail on the scratches?
No.

I cleaned and reassembled the newly aquired used caliper now and put it all back together, added new brake pads and bled the lines.

Since my original brakes weren't leaking and the brake pads might just have reached the end of their life I don't feel terrible about it.

Your original looks better, at least in the photos viewed from the other side of the planet. I don't know what they feel like, though.
The old one looks more scruffy but the single scratch in my OEM ones was actually worse.

I will go for a test ride now and tightly observe from now on for any binding or leaks. It's not a perfect solution but I don't expect catastrophic failure.

Edit: went for a ride, new brake pads feel great compared to metall on metall 🤓, rotor warm after riding, seems ok for now will see how it develops.
 
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