choddo

Hi all

does this look like a stuck piston? Or could something else cause this? I was able to push both pistons back without problems.

Iride01 

And the pad in the rear is what was on the opposite side of the one in front?

Hard to believe the wheel rolled free enough to not notice some drag. I've got nothing for you. Just curious how there could be that much disparity between the two sides.

What brake brand and model are these pads from?

choddo

Yep. Outside (closest to camera) of rear wheel vs inside.

No significant drag, been remarkably noise free actually - a little bit of scraping recently which was why I took a look.

Shimano GXP calipers and Shimano L05A-RF pads - probably about 18 months old. I replaced the fronts in April and just had a look at those and they're about half worn, and evenly.

FBinNY 

If this is a mechanical brake with 1 moving & 1 static pad, you may have it too far off center.

The way these work is the moving pad pushes the rotor over against the static pad. So the disc's rigidity creates uneven forces. The stiffer the rotor and the farther it moves, the worse this is.

Try centering the brake so the static pad has minimum acceptable clearance.

OTOH it might be as simple as one side of the rotor being smoother than the other.

Iride01 

I'd have to think that the pad in the rear of the picture must have been stuck for some reason. Whether that was from the piston being stuck or just something that had it wedged on it's slider or in tiny amount of space it has in the brake body preventing it from engaging the rotor.

Was there a lot of gunk or trash up in the brake body on that side for the one that isn't worn?

Alan K

When you change your pads, set the clearances/gap between the rotor and pads, and angle of the pad block right ensure that rotor and pads are perfectly parallel. It’s a good idea to give your disc brake system a quick look periodically and check screws holding the calipers etc to ensure that they are firmly in place. If some of these screws become slightly loose, the entire assembly may shift slightly which forces the rotor to flex every time you brake. Brakes will work suboptimally, one pad will wear more than the other, and a rider may or may not notice too much difference in braking power - most of us tend to feather brakes most of the time and hard braking is rarely used.

FBinNY 

FWIW I can't imagine a stuck pad causing this without very noticeable drag and/or noise.

So look for something that happens when the brakes are used.

choddo

Yeah seems like it.

Not much, I think just typical. I was brushing it down after each ride and it looked ok.

choddo

Thanks - this caliper is rock solid so I don't think it's moving. I think I properly aligned it a few months back when I bled it. And I cleaned and lubed the pistons with mineral oil. Maybe I got something wrong then but it's been working ok since as far as I could tell. It span cleanly and no noise when braking until very recently, like the last 150km maybe 3 road rides. The weather has been abysmal for those.

Alan K

Let’s hope then that when you disassemble everything, clean and relubricate moving parts (including pistons) when you change pads, everything will work fine. [You may also want to check that rotor(s) are perfectly flat as they should be.]
Some ailments of bikes can be cured by simple disassembly, clean up and reassembly.
Good luck!

Turnin_Wrenches

Did we ever find out of the OP had mechanical disc brakes with only one moving piston? If so, the explanation provided by FBinNY makes sense. I'm not on board with the other hypothesis that one side of the rotor might be smoother than the other. IME any slight roughness on a rotor will make noises that get your attention.

I'm interested to learn how this saga ends...

Turnin_Wrenches

In response to the portion in bold above... . I'm amazed how often that works. Basic, regular maintenance and cleaning is a beautiful thing for keeping things running smooth, for keeping things properly adjusted and aligned, and for fixing and preventing noises you'd rather not hear while riding your bike.

As for checking that your rotors are "perfectly flat", good luck with that! Spoiler alert: they're not. I've never met a rotor perfectly in true, and I have long abandoned all hope that any exist (or ever will) . With effort you can get pretty close.

Another common problem that can make disc brake setup tricky is brake mount surfaces that aren't flat. Many bike manufacturers do a poor job when it comes to facing brake mounts on a frame, so you're often at a disadvantage before you even get started. The uneven mounting surface causes the brake caliper and brake pads to sit at an angle in relation to the rotor, which can cause brake rub and uneven wear to the brake pads.

choddo

No it’s hydraulic.

I think the brake mount is ok. It’s a Trek Checkpoint. I didn’t have too much trouble aligning it. I’m wondering if, when I cleaned the pistons last time, I pulled the brake too hard and caused an issue with the travel of the piston on that side - I remember it not looking quite parallel with the caliper housing as I was rubbing some mineral oil on it - even though I was able to push it back in without a problem and the brake seemed to work fine for the last few months since I did it. I’ll have a closer look in the next couple of days and clean it all up again properly.

Kontact 

You don't need a stuck piston for one piston to move more than the other. That's a seal friction issue, and you can usually work it out by pushing them both in, holding the one that moves more in and pumping out the other one.

Iride01 

If you get to fiddly with trying to investigate and fix issues that really aren't the cause, then you might just make another problem.

If you cleaned it up and put it back together, then just use it and check the pads sooner to see if they appear to be wearing on both sides. It might have just be a very small piece of something that jammed the pad or it was slightly twisted and jammed. Or as Kontact said, one piston may not have been working for similar reasons. But since you've moved them and cleaned things up, it might be all okay now.

choddo

I’ve half done the job. New pads went in fine but rubbing a little in the existing caliper position so they’re coming out again and I’ll clean the pistons properly and then re-align. Fingers crossed. Used SwissStop Disc34RS this time though didn’t realise they don’t have cooling fins or anything cyberpunk like that so I’ll probably assplode the rotors next descent.

FBinNY 

FWIW a bit of (simplified )brake tech here.

Regardless of any effort to cool the pads, the vast majority of brake heat flows to the disc. That's because the pad material is more of an insulator than the steel disc. Also because heat always flows hot to cool, and the moving rotor is continually bringing in fresh, cooler areas.

That's why rotor heat and air cooling capacity are the biggest factors in overall brake performance. Also, if any significant heat flowed to the caliper, it would kill the seals and/or boil the brake fluid.

choddo

My physics degree should have told me that.

Still like the Gigeresque fin design though.

choddo

Cleaned them all up properly and it looks like they were pulling evenly to me. Took it out this morning, those SwissStop pads work really well. Cruddy out on the roads again so they were put through their paces. Will be interesting to see if this issue develops again, will check in a few weeks. Thanks to everyone who commented/advised.

GamblerGORD53

The pads are wearing straight. Rotate them if it does the same thing.