Stoenr's right. It's normal. Look at any car's brakes after it rains. You'll see pretty much every car has it. I think it has to maybe do with the water mixing with loose brake dust which turns it a brownish color.

Stoenr said:

Mine have a thin thin layer of rust by the middle of washing. It happens that quick. But like he said ^ its gone after a few brake applications. Normal for any car.

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