First off- I wouldn't worry about it, brakes rust.... it's normal.
Second off- Just go do a nice hard braking session. Then get some plain wet-wipes or something to take the dust off.
If your talking about rust that has built up a little, you really can't get it off and if you do all you are doing is exposing new material to rust.
It's better off that you just leave it alone, clean the "show off" parts of your car (Wheels/brake rotors) and be done.
2011 Mustang GT, Black with Black stripe.
6 Speed Manual.
3.73 + Brembo Brake Package.
Charcoal Black Interior.
Appearance Package 4.(Hood Scoop and Rear Spoiler)