Caliper problem? Inner pad worn when outer pad unworn

[ATB88]

Recently got a loud grinding noise coming from my front right brakes when braking. Popped the wheel off to investigate, and found that the inner pad was worn down to the metal, which was scraping the rotor. Interestingly, the outer pad has *tons* of life left, as do the brakes in the other three corners. FWIW this is a drilled BMW ZCP front rotor with BMW ZCP pads.

I'm wondering what would cause this? Bad caliper? Or maybe just improperly bled? I'm replacing the rotor and pads, but want to make sure I address the root cause so it doesn't happen again. How should I go about further diagnosis?

[bigjae46]

I would imagine if there was a caliper issue then the car would pull when braking. Maybe ABS has been hiding the issue? I'd inspect the guide pins and contact points, make sure there is evidence of binding.

I'm a fan of finding out what the issue is. BUT...given the age of these cars it is not a bad idea to consider replacing the guide pins, dust boots and pressure seals on the calipers. I believe some kits come with a new piston. If your brake lines are original, consider replacing those as well. Be sure to clean up the caliper and apply anti-seize to any contact points.

If you do rebuild the caliper, buy an AN male plug to screw into the end of the hose while the caliper is off. Makes life much easier.

[ATB88]

Yeah good point. At this point, it wouldn't hurt to just rebuild the caliper. I'm a little busy to take that on myself at the moment, though. Anyone know if there a fast reliable rebuild service being offered somewhere? All the better if they're in SoCal...

Also, I'm currently having my DSC light come on often during normal driving. Scanning for codes, I get a brake fluid level sensor code. The reservoir is full, so I'm guessing that it's just the sensor itself that's faulty. This seems like a separate problem to the above, but if there's a way they could be related, would be good to know!

[sapote]

and found that the inner pad was worn down to the metal, which was scraping the rotor. Interestingly, the outer pad has *tons* of life left,

The inner pad is on the piston side. This means the piston is not retracting freely. Remove and clean piston and replace dust boot on piston.

[sapote]

Normally outside pads should wear faster than inner ones.

[Pklauser]

I get pretty uneven pad wear on track with new brass guides, new pistons, and new seals. It also seems to change event to event, eg. one day I'll get pretty even wear, the next it'll be uneven again. Haven't gotten to the bottom of it, but I also may not be seeing as extreme of a case as you are.

[Estoril]

I would rebuild the caliper (actually both on the same axle). Then fully clean it, inspect and clean caliper pins and replace any that have rub marks. Caliper piston should show no rub marks - if so, replace. Replace seals and lube piston on reinstall.

[eacmen]

As others have said I also suspect seized slide pin. The piston is able to push the inner pad but isn’t able to move the outer pad.


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[sapote]

As others have said I also suspect seized slide pin.

In this case, the outer pad should wear more than inner one as the whole caliper not able to slide outward freely, causing the outer pad dragging on the rotor.

[eacmen]

In this case, the outer pad should wear more than inner one as the whole caliper not able to slide outward freely, causing the outer pad dragging on the rotor.

If the caliper is totally seized in the slide pins it won’t move inboard or outboard. So the outer pad wouldnt drag and wouldnt be used. However the caliper piston is free to push the inboard pad.


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[sapote]

If the caliper is totally seized in the slide pins it won’t move inboard or outboard. So the outer pad wouldnt drag and wouldnt be used. However the caliper piston is free to push the inboard pad.


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I see, but this has never happened on this world, unless a welder had worked on it

[BTB]

Yeah good point. At this point, it wouldn't hurt to just rebuild the caliper. I'm a little busy to take that on myself at the moment, though. Anyone know if there a fast reliable rebuild service being offered somewhere? All the better if they're in SoCal...

Also, I'm currently having my DSC light come on often during normal driving. Scanning for codes, I get a brake fluid level sensor code. The reservoir is full, so I'm guessing that it's just the sensor itself that's faulty. This seems like a separate problem to the above, but if there's a way they could be related, would be good to know!

You can get rebuilt calipers from ECS, Turner etc. for ~$100 after core exchange. Ref link below

https://www.turnermotorsport.com/p-366888-brake-caliper-rebuilt-front-right-e46-m3-csl-z4m/