I decided to rebuild my brake calipers by buying cheap used ones and a complete rebuild kit (all the seals + piston), so I can do it off the car, paint it and just swap it later. The ones I bought were actully in perfect condition inside.

Unfortunately, my drill with a steel brush slipped during cleaning and entered the cylinder, even though it was already stuffed with a rag it still did scratch the cylinder wall a bit. Obviously due to rotation, the scratch is not along the piston travel path but perpendicular to it. It's not deep, I cannot feel the groove under my nail but under my finger I can feel the area there is not that smooth. Piston still travels nicely though.

I do not worry about leaks, as the scratch is below the rubber seal. I'm only worried if it won't cause me some problems with the operation of the new piston. I saw this site: https://blog.frenkit.es/en/how-to-clean-rust-inside-a-brake-caliper and they just literally say to just sand down rust inside the caliper... should I attempt to do the same or just live with it? The caliper is an aluminum cast one, to be honest I'm kinda more worried about sanding it down than leaving it as it is.

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  • Your link is about rust on the inside of the caliper, not the piston. Feb 25 at 18:32
  • @WeatherVane yes and thats exactly what i'm talking about here too - the inside of the brake caliper got scratched, piston is brand new from the repair kit
    – Ve1zy
    Feb 25 at 20:01
  • Sorry I don't understand. You wrote "steel brush slipped during cleaning and entered the cylinder, even though it was already stuffed with a rag it still did scratch the cylinder wall" Feb 25 at 20:03
  • Yes, at least to my understanding the cylinder is the "hole" that the piston works inside of. So I did not scratch the chromed shiny piston that pushes the brake pads out, I scratched the housing of that piston because I started cleaning the caliper after removing the piston (which was probably a bad idea)
    – Ve1zy
    Feb 25 at 21:08

1 Answer 1


Leaks should be your #1 worry with brakes. Leaks means loss of fluid, which could lead to loss of brakes. That is terribly worrisome to me. Secondly, the problem with the scratch is, if the piston seal is running across the scratch, it will tear up the seal. Considering the piston seal pulls double duty as both seal as well as the retraction mechanism for the piston itself, this means it could cause the piston to drag and not fully disengage from the piston bore.

You may be in luck if the scratch isn't too deep. You DO NOT want to run sand paper across it, because as hard as you might try, this will get the cylinder out of round and that's probably worse than the scratch. What you need to get is a hone used to recondition the piston bore. Depending on the depth of the scratch, it shouldn't take much more than a couple of passes to get it back to looking beautiful.


Considering the new cutaway which you provided, I'm doubting you're going to see any issues at all. If you take something like sandpaper and take the high edges off the scratch, it would probably be best. As you were saying, there's no seal which would rub against it. The only thing I'd be worried about is if the scratch in the caliper body created a scratch in the piston. If you do take down the sharp edges of the scratch, ensure you've cleaned the caliper body REALLY well. You don't want grit or metal remains to be in there, because that definitely would cause you issues with tearing up the seals.

  • I may have written something incorrectly as I'm not a native english speaker, but I do not have any scratch on the piston itself (the part that moves out). In fact I have a brand new one. The scratch was made on the inner wall of the caliper itself, where the piston is seated. The rubber seal sits stationary there in a groove and it's way above the scratch. Unfortunately I didn't take any photo so I could attach it to the main post
    – Ve1zy
    Feb 25 at 20:02
  • @Ve1zy - Your description is exactly as I understood it. The problem is the cylinder wall which the piston rides in ... if there is a scratch there, it can cause the seal on the piston to catch on it and damage the seal as the piston pulls it back and forth across the scratch. Feb 25 at 22:58
  • I don't quite understand how the seal could catch on the scratch, as it sits stationary in the groove on top of the cylinder. Piston itself has a little clearance and can be "wobbled" around until the seal is in place. I have attached a picture of the scratches to the original post, don't mind the debris inside though.
    – Ve1zy
    Feb 27 at 19:00
  • @Ve1zy Paulster2 is right. Damage to the interior surface of the wheel cylinder bore (that's the cylindrical space in which the piston moves) is a classic recipe for tearing up the seal. As the piston moves in the bore, the seal is dragged across the scratches and gouges. Unless remedied, there's a significant risk that after some use, the piston seal will fail with no warning: the brake pedal will go to the floor, and much or all of the car's braking force (depending on the system's design) will be lost. I would not drive a car with that damage. Feb 27 at 20:34
  • @DavidRecallsMonica I still somehow cannot understand how would the seal be dragged across these scratches, at least in this particular brake caliper the seal sits stationary in the groove located inside the caliper itself (visible on the photo I added). The piston slides on the seal, but it's not the piston that's scratched - I have a brand new one.
    – Ve1zy
    Feb 28 at 20:59

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