When the brakes are applied to slow down the vehicle heat is produced. This heat may cause the brake rotors to warp or buckle. If the brake discs are warped even three or four one thousandths of an inch, the brake pedal may pulse and the vehicle may shake when the brakes are applied. If the brake discs are badly warped the whole vehicle may shake and cause the steering wheel to oscillate and shake from side to side under braking.To eliminate this unwanted vibration the brake rotors need to measured both for uniform thickness and “run out”. Run out is a measurement by rotating the brake disc and accurately measuring how much deflection the surface of the brake disc takes as the disc is rotated. The brake discs are brought back to uniform thickness and parallelism by machining the brake discs if there is sufficient material left to remove. If the brake pads have worn down to the point where they have damaged the brake discs the brake discs will need to be machined or replaced.
Thanks, That explains it. The rotors had been cut before and obviously have warped now. it's my wife's car and she had them cut without my knowledge. I told her since the price to cut is the same as to replace, always replace the rotors.