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I got an as-is bike used that came with disc brakes. Both brakes squealed so I cleaned both rotors and both sets of pads. That fixed the back one but not the front one. I removed the pads and noticed that there is something that looks like grease in the holes of my front rotor. I used a q-tip and rubbing alcohol to clean both the pads and rotor and that quiets it down for a short time, until the grease re distributes over the rotor surface.

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    maybe i'm paranoid. but if i thought there had been grease on my pads and rotor, i'd immediate replace both – Paul H May 16 '19 at 17:34
  • You could maybe try using a pipe cleaner and a de-greasing solvent to clean out all the crannies of the brake? Or replace it... – DavidW May 16 '19 at 17:47
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    A picture if the rotor with the offending substance will probably help. It may just be dirt, not actual grease. – Argenti Apparatus May 16 '19 at 18:29
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    The Grease is coming form somewhere. Find out the source and fix that, then clean the rotors and replace the pads. You will be best to remove the rotor to clean it, although the amount of effort required to know for certain its clean, a new rotor might be worth considering. – mattnz May 16 '19 at 21:39
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    You've tagged this with hydraulic-disc-brake so the main source of greasy fluids would be brake fluid leaking around the pistons. Possibly then discoloured by dirt. And your pads are contaminated too - which need replacing once the source of the leak is fixed. – Criggie May 17 '19 at 7:55
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I'm sure someone will be able to provide a better answer before too long, but my solution for this sort of situations is baby wipes. I've used them on various parts of bikes (and babies) and they work really well - I think it's mainly thanks to their ability to hold/absorb substances. Things to bear in mind:

  1. We use pampers sensitive and I assume most brands will give you similar results, but I do note that pampers sensitives are fairly flossy in texture whereas some brands like huggies are more cloth like. Hope it's ok to use brand names on stack exchange?
  2. I imagine it may take some experimentation before you find a way of dealing with the shape of the rotor, feeding wipes through holes, getting into awkward corners etc.
  3. You may also find they leave some floss on, it might be worth clearing that so that you don't jump from the frypan into the fire - the wipes may have not been tested under the pressure and temperature present in disc brake systems. If the floss is synthetic it could melt under heat and become harder to remove.
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    I doubt somewhat that baby wipes (!) are decent degreasers. Degreasing is not exactly what one might wish for a baby's skin. Also, floss contamination should not rank among my most pressing concerns when riding on a bike with a greasy brake rotor. – gschenk May 18 '19 at 0:30
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    Doubt is the usual first reaction I get when I suggest baby wipes in these sort of situations, and indeed it was my reaction when I was first advised to use them. As above, I really think it's their ability to absorb dirt etc that works so well in mechanical situations like bikes. I edited my answer and added why I'm concerned about floss. – pateksan May 18 '19 at 0:42
  • While I doubt they are ideal in a shop compared to a rag with some more serious solvent, of things one might have along on a ride they don't seem the worst choice. If I had nothing more suitable at home I might use Isopropyl Alcohol on a paper towel - but on the metal components only. – Chris Stratton May 18 '19 at 3:17
  • I've found even the sensitive baby wipes to be good for getting grease off hands, but they'd be likely to leave a slippery residue on brakes. Glasses wipes are much better for degreasing, as they're wet with isopropyl alcohol and water (and they come individually wrapped for taking on rides, as do pre-injection wipes which are smaller) – Chris H May 19 '19 at 7:34
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    Thanks. I took the baby wipes suggestion and tried it. Well, they were actually makeup remover wipes but they worked like a charm. My brakes are 100 percent quiet now. I also cleaned off where the hoses connect to the caliper which did have some built up grime of the same type as the rotors on it. – Owen Marsh May 19 '19 at 16:22
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The grease-like substance on the rotor holes could be from the brake pad when you squeeze on the lever. I suggest buying a fresh set of pads.

As for the rotors, dumb them in a tub of degreaser (bike-specific or industrial). After a while, get a toothbrush and scrub away at every surface and corner of the rotor while submerged in degreaser (wear gloves please), then leave it for a bit more.

Rinse the rotor with water then wipe with isoproryl alcohol and leave to dry. Dispose of degreaser properly or get a coffee filter and recycle it.

Attach rotors back to the hubs (don't touch the braking surface), install new pads, and see how it feels.

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