The rotors need more than a visual inspection. First they must be measured for thickness variation. That means measuring the thickness of each rotor in a minimum of six places and comparing them. A difference of only a couple of thousandths of an inch is enough to feel in the brake pedal.
Next they must be checked for lateral runout. That involves running it in gear with a dial indicator on the rotor's braking surface. Every rotor will have a little runout. It's a matter of how much is enough to cause a noticeable tugging on the steering linkage.
The first type of warpage is when the two braking surfaces have a different thickness at different places around the rotor. As the fatter section passes through the brake caliper, it pushes the piston back in which pushes brake fluid back up to the master cylinder and pushes the brake pedal up against your foot pressure. The lateral runout means the parallelism can be perfect all around the rotor but the entire assembly is tilted relative to the center mounting hub. That makes it wobble as it rotates and tugs the steering wheel back and forth. That can have absolutely no affect on the brake pedal but usually you will feel some pedal pulsing.
Friday, June 3rd, 2011 AT 7:24 PM