To clarify the rotor issue, there is a published legal minimum thickness they can be and no mechanic will risk his reputation or the chance to sit in a courtroom from leaving them on the car knowing they're too thin. We all know a little bit undersize won't cause a problem but a good lawyer or insurance investigator WILL convince a jury you were partly responsible for the crash when the other guy ran the red light because you were less able to avoid the crash. We all know that's hogwash, but you can't win against lawyers.
At the very least you want to have the old rotors machined to true them up and remove any warpage. Failure to do that will result in uneven braking until the pads wear down to match the grooves that ARE worn into the rotors now. That can take a long time. I can't speak directly to a Lexus, but in general, new rotors today are so inexpensive that most shops find it less expensive for you and for them to just replace them during any brake job. When you factor in the cost of the cutting bits for the brake lathe, and the mechanic's time to measure, inspect, machine, and clean the rust and debris from the mounting surface, they save you more money in the long run by replacing them.
There are a lot of things experienced mechanics do to resolve noise issues, prevent causing noise issues, insure long life of the brake components, and avoid vibrations that most do-it-yourselfers don't know about. Many of those things involve cleaning and lubricating the rotor. Cleaning takes a lot of time and they can still miss things. That's another reason they prefer to replace them with new ones.
Be aware too that there are a lot of Chinese rotors available that usually cause a pulsation a month or two after installation. There is nothing wrong with the quality, but when we make parts out of cast iron, we let them "age" for 90 days before we do the final machining. They cast, machine, pack, and ship their rotors right away, then they age on your car. Usually one followup machining a few months later is all that's needed to solve the pulsation permanently.
Monday, October 22nd, 2012 AT 12:44 AM