This is much more common with new rotors a few months after they were installed. Used ones that were machined don't usually do that. There's two things to consider. First of all, there is a published legal minimum thickness they can be machined to. Going thinner than that will rarely cause a problem but no professional will risk his reputation or sitting in a courtroom explaining his actions by going beyond that limit. Unless an untrained and inexperienced beginning mechanic machined them too far, that is not likely the problem. Going too thin can cause them to heat up faster and possibly warp.
You also have to look at a dragging brake. When you shift to neutral on a slight incline and release the brakes the car should creep downhill on its own. If it does not, after a short drive feel which wheel is hot. Three things can cause a brake to drag; the caliper, the rubber flex hose, or the brake fluid is contaminated with a petroleum product. The last one isn't likely until everything else is ruled out. If you crawl underneath and open the bleeder screw on the caliper and it doesn't release, replace the two front calipers for even braking. A ring of rust or dirt can build up around the piston when the old pads wear down. To make room for the new thicker pads those pistons have to be pushed back in and that pushes the dirt under the square-cut seal causing them to stick.
If the caliper does release when the bleeder screw is opened, the rubber flex hose is the suspect. Rust can build up inside one of the crimps and constrict the hose, or the inner liner can be torn if the caliper is allowed to hang by the hose.
A sticking brake will cause that thumping when braking, but if the brakes seem to be releasing normally a rotor may just have decided to warp. Machining or replacing it will take care of that.
Wednesday, May 8th, 2013 AT 7:14 PM