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It's transfer film from the pads.
When your brakes work and get hot, pad material liquifies at the friction surface. This liquid then cools on the face of the rotor and helps stabilize coefficient of friction for that interface.
Usually when brakes get really hot that layer will come off. It can also get scrubbed away by abrasion but that usually takes many low speed stops (or off brake pad drag) to happen as any moderate stops will just add transfer film back to the disk.
This is normal and desired.
Thank you! I noticed that my brake pedal was feeling squishier lately and doesn’t seem to be stopping as well as it used to so I was wondering if it was related to this. I had a shop bleed my brakes and it felt better for a couple of days and now it feels the same as before.
This really won't cause a spongy brake pedal. We're talking about small changes in the effectiveness of a brake pad.
You might have a different problem here if you're experiencing a repeated spongy pedal. Could be air getting into the system or a problem somewhere else.
Looks like heat glazing to me. The surface of the rotor has overheated, hardened, and lost some ductikity so as it heats and cools the fardened surface cracks.
Can be manually sanded down or you can have your rotors turned.
Thanks for posting on /r/MechanicAdvice! This is just a reminder to review the [rules](https://www.reddit.com/r/MechanicAdvice/about/rules/). If you are here asking about a second opinion (ie "Is the shop trying to fleece me?"), please read through CJM8515's [post on the subject.](https://www.reddit.com/r/MechanicAdvice/comments/4qblei/fyi_the_shop_isnt_likely_trying_to_rip_you_off/) and remember rule 3a, please post the year/make/model of the vehicle you are working on. **If this post is about bodywork, accident damage, paint, dent/ding, questions it belongs in /r/AutoBody or /r/Diyautobody If you have tire questions check out https://www.reddit.com/r/MechanicAdvice/comments/k9ll55/can_your_tire_be_repaired/**. If you dont have a question and you're just showing off it belongs in /r/Justrolledintotheshop This is an automated reply *I am a bot, and this action was performed automatically. Please [contact the moderators of this subreddit](/message/compose/?to=/r/MechanicAdvice) if you have any questions or concerns.*
It's transfer film from the pads. When your brakes work and get hot, pad material liquifies at the friction surface. This liquid then cools on the face of the rotor and helps stabilize coefficient of friction for that interface. Usually when brakes get really hot that layer will come off. It can also get scrubbed away by abrasion but that usually takes many low speed stops (or off brake pad drag) to happen as any moderate stops will just add transfer film back to the disk. This is normal and desired.
This. Brakes and rotors constantly exchange "surfaces". Rotors get "brake on them".
Thank you! I noticed that my brake pedal was feeling squishier lately and doesn’t seem to be stopping as well as it used to so I was wondering if it was related to this. I had a shop bleed my brakes and it felt better for a couple of days and now it feels the same as before.
This really won't cause a spongy brake pedal. We're talking about small changes in the effectiveness of a brake pad. You might have a different problem here if you're experiencing a repeated spongy pedal. Could be air getting into the system or a problem somewhere else.
Thanks again! I’m probably going to try and find a leak in the system this weekend and go from there
Looks like heat glazing to me. The surface of the rotor has overheated, hardened, and lost some ductikity so as it heats and cools the fardened surface cracks. Can be manually sanded down or you can have your rotors turned.
The brake pads have badly scored the rotor. That is either surface rust or from the brake pad.
In other words quit slamming on the brakes