To my untrained eye, I would say that water probably leaked into the cap, did it have a rubber O-ring around the outside? The rotor looks like it just has normal wear

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TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth