If the brake pad hasn't been pusing on the disc on that side, that brown ring you're seeing could very well be rust. Given the type of use brake discs see, there's no reason to rust-proof them.

Just a thought...good luck!

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2001 VW Jetta TDI, 5 speed, daily driver
1991 Ford F-350, work in progress
1984 Ford F-250 4x4, 6.9l turbo diesel, 5 speed manual
Previous oilburners: 1980 IH Scout, 1984 E-350, 1985 M-B 300D, 1979 M-B 300SD, 1983 M-B 300D
Spark-free since 1999