Now that the days are getting longer and I can do critical service work without a flashlight in 9 degree weather, I can see a brown ring on my brake rotor. The caliper failed on this side.
I dont think my failed brake caliper siezed on the rotor and I dont think my brakes were stuck in a locked braking position and the brakes were not literally smoking.
I think the brake piston got stuck in the retracted postion and ceased to function and provide braking pressure.

Nevertheless my Rotor has a brown ring on it. Should this be indicative of a failed rotor and thermal discoloration? Or is the brown ring a sign of some less severe condition. I always though thermal discoloration of overheated high carbon steel was Blue and not brown...

Brown discoloration on Brake Disk Rotor 83 240D

See if it comes off with some steel wool - e.g. organic contaminant or rust.

Steve

Discoloration from heat is anywhere from straw to black. The stages are: light straw-straw-purple-blue-black. Brown is hydrated iron oxide. I think you're looking at rust.

Does the car sit idle for long periods? Brakes suffer first on a car that sits unused. If the shiny metal doesn't come back on the rotor surfaces after running it for a couple weeks, then check into the possibility you may have a seized caliper.

__________________
95 E320 Cabriolet, 159K

Wash your car & then park it right away.
Look at the rotors the next morning, and you will see the same brown ring ( rust ).
Not much to be concerned about, as you will scrub it off with your first brake application.

__________________
2007 C 230 Sport.