If you hear a single clunk when applying the brake pedal, that is usually due to missing anti-rattle hardware on a pad or caliper, but when it's once per wheel revolution, it's usually a combination of a couple of things. First, there is likely a groove worn into the anchor the pads rest against when the brakes are applied. That is caused by a failure to use high-temperature brake grease on the contact points, and normal wear. Most of the time that doesn't cause a problem unless the second problem occurs. That is anything that causes the caliper to slide back and forth. That can be a warped rotor or from debris stuck between the rotor and hub. New rotors can be warped from rough handling in shipping but most commonly, Chinese rotors warp after a couple of months. It has nothing to do with their quality. It has to do with how they're "aged". We cast parts from cast iron, then let them sit for 90 days before we do the final machining. The Chinese cast their parts, then pack them and ship them. They age on your car, and that can result is minor warping. A simple light machining will take care of that.
With a new wheel bearing / hub and a new rotor, stuck debris is going to be less likely. It's very common on GM cars when old rotors are reinstalled. There's three access holes in the hub where water and salt can splash up onto the backside of the rotor. That results in three round spots of raised rust. That rust has to scraped off before the rotor can be machined, otherwise it will wobble on the lathe and a warp will be machined into it. If the rotor is not machined, that rust isn't cleaned off, and it's reinstalled in a different orientation, that rust will be clamped between the rotor and hub. You may not feel the wobble in the steering wheel or brake pedal, but it can make the caliper slide back and forth with each wheel revolution. Couple that with that groove I mentioned and it can set up a clicking noise.
The way I look for this is to use a dial indicator against the wheel near the edge where the weights go. If there's very little lateral runout there, chances are the rotor is okay too. If you do find some runout, remove the wheel, reinstall the lug nuts, then measure the runout right on the rotor. Usually if there's enough runout to cause the pads to click, you'll be able to see it on the wheel without needing a dial indicator. Harbor Freight Tools has an inexpensive dial indicator and base that work fine for this.
Thursday, November 14th, 2013 AT 11:11 PM