It could be what this is talking about
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Surface crazing or cracking on brake discs
The picture above shows a normal cast iron brake disc that exhibits what we call crazing. These are tiny hairline cracks that are extremely small and not of real concern. These are microns deep and are exaggerated by the black brake dust generated under normal driving. This does show that the brake discs have been quite hot and could encourage you to have your discs Pro Cut skimmed which can be performed several times during the life of your brake discs or rotors.
The Pro Cut skim removes only 4-6 thousandths of an inch and does not cause the discs to go below thickness.
There is good logic for this type of “Renovation” of your brake discs rather than replace the discs with new ones for the simple reason that a good USED disc is actually a better performing product than a new one. New discs require lengthy bed in times during which time the disc materials “Settle” and grain structure alignment takes place. Also the Pro-Cut machine is brilliant at taking a minimal skim from your discs WITHOUT REMOVING THEM FROM THE CAR which exposes you to a re-seating process and possible disc distortion and vibration arising.
......from ...... the internet......