That is due to a warped brake rotor and is very common on any car. (Reread number 4 in my previous reply). The fix is to have it machined which is also a normal part of any regular brake service.
I made some sad drawings to show the two kinds of warpage. In the left drawing, the mounting surface, (blue arrow), and the two sides of the friction surface, (green and red arrows), are all perfectly parallel. That's necessary for smooth braking.
In the middle drawing, the friction surfaces are still parallel but they are not parallel to the mounting surface. This is what happens from uneven heating and cooling, especially splashing cold water up onto the rotor when it is hot, and that will be greatly aggravated by uneven lug nut tightness. The brake caliper will slide sideways back and forth for every wheel revolution and will tug on the steering linkages. You'll feel that vibration in the steering wheel and seat but not in the brake pedal unless the warpage is severe.
In the right drawing the two plates, or sides of the friction surface are not parallel to each other. As the rotor goes around, the brake pads have to move in and out to maintain contact. When the thicker area comes around, it pushes the brake pad and piston back into the caliper which pushes brake fluid back up to the master cylinder where it pushes the brake pedal back toward you. You'll feel that as the brake pedal rising and falling with each tire revolution. It will often be felt in the steering wheel too but the brake pedal gets all your attention. These drawings show everything greatly exaggerated. It only takes a few thousandths of an inch thickness variation to be felt. That's less than the thickness or two sheets of paper.
There is a legal minimum thickness a brake rotor can be machined to, and every manufacturer publishes those numbers. It can be allowed to wear to a certain thickness, but the "machine to" specification is usually.030" thicker. That means a rotor can be thick enough to legally remain on the car but not thick enough to legally machine it to remove the warpage. In that case they must be replaced. Fortunately new rotors usually aren't very expensive.
There are a few tricks professionals do when installing new rotors to reduce the chance for noises and vibrations but often new rotors will still warp in a few months and have to be machined. This is more common with the less expensive Chinese rotors. There's nothing wrong with the part itself but when we make parts here out of cast iron, we let them "age" for about 90 days, then perform the final machining. Chinese parts are cast, machined, packaged, and shipped right away, then they age on your car and often warp. One machining is all that's needed, then they typically don't cause a problem after that as long as you wait long enough for all the warping to occur before having them machined.
Friday, March 16th, 2012 AT 5:13 PM