Bluing or Heat Checking
If the lining surface is charred, blued, or hard-ended with a heavy glaze, or if the rotor is severely heat checked, machine the rotor or replace it.
Hard or chill spots of steel in a rotor cast-iron surface usually result from a change in the metallurgy caused by braking heat. Pulling, rapid wear, hard pedal, and noise occur. These spots can be removed by grinding; however, only the raised surfaces are removed, and they could reappear when heat is again encountered. The rotor must be replaced.
Lack of Parallelism
Parallelism refers to variations in thickness of the rotor . If the rotor is out of parallel, it can cause excessive pedal travel, front-end vibration, pedal pulsation, chatter, and on occasion, grabbing of the brakes. It must be resurfaced or replaced.
To check a rotor's parallelism, measure the thickness of the rotor at eight different spots. Courtesy of American Honda Motor Co., Inc.
Excessive lateral runout is a wobbling of the rotor from side to side when it rotates. The excessive wobble knocks the pads farther back than normal, causing the pedal to pulse and vibrate during braking. Chatter can also result. It also causes excessive pedal travel because the pistons have farther to travel to reach the rotor. If runout exceeds specifications, the rotor must be turned or replaced.
Checking the lateral runout of a brake rotor. Courtesy of American Honda Motor Co., Inc.
If the vehicle has not been driven for a period of time, the discs rust in the area not covered by the lining and cause noise and chatter. Excessive wear and scoring of the discs and lining result. Wear ridges on the discs can cause temporary improper pad lining contact if ridges are not removed before installation of new lining. Lining deposits on the disc can cause erratic friction characteristics if a new lining is installed without resurfacing or cleaning the disc.
Rotor wear or scoring can be caused by linings that are worn through to the rivets or backing plate, or by friction material that is harsh or unkind to the mating surface. Rust, road dirt, and other contamination could also cause rotor scoring.
Light scoring (less than a depth of 0.015 inch 0.381 Mm) of the disc braking surface can occur during normal brake use. This does not affect brake operation; however, it can result in a higher-than-average brake shoe lining wear rate. But, any rotor having score marks more than 0.15 inch (3.8 mm) should be refinished or replaced.