Hi bobcopenhaver. Welcome to the forum. A bent hub is always a possibility but they are pretty tough. If you installed new rotors, don't overlook the possibility they are bent too. Blame it on rough handling during shipping.
If you installed used rotors or when your old ones are machined, you must check for rust spots on the mounting surface of the rotor. There is at least one bolt access hole in the hub. Water and salt sneaks in there and causes rust buildup inside that hole on the back of the rotor. If it is not scraped off, the rotor can wobble on the brake lathe and a warp will be machined into it. If you install used rotors, be sure to clean those rust spots off.
If it took a few weeks for the pulsation to show up after new rotors were installed, it is possible they are Chinese parts. There is nothing wrong with the way they're made, but cast iron parts made in the U.S. Are set aside for 90 days to age before the final machine work is done. Chinese parts are shipped right after they're made, and they age on your car. It is real common for them to warp after a month or two. Usually a single light machining will be all that's needed to permanently solve the problem.
If none of these simple things apply to your situation, it's time to attack the problem with a dial indicator. Start by running the engine in gear while the front end is jacked up and supported on jack stands. Hopefully when viewing the tire from the front, you won't see and sideways run out. Place the dial indicator on the lip of the wheel near where the weights go. If the reading is low, typically less than.040", that will eliminate most items except the rotor itself.
Next, remove the wheel, reinstall the lug nuts, and use the dial indicator on the braking surface of the rotor. If you find more than about.010" run out, use the dial indicator on the hub surface. If the hub is true, look for debris or scale that got trapped between the rotor and hub. If everything is clean, suspect a warped rotor. Proof will be if the reading is much lower on the rotor mounting surface that contacts the wheel. You'll have to make a judgment call on how much run out is too much. As you get closer to the axle with the dial indicator, the amount of run out that can cause a problem becomes less.
A real common cause of multiple warped rotors is failure to use a torque wrench when tightening the lug nuts. Uneven clamping forces coupled with heating and cooling cycles causes the warpage, but it usually doesn't show up right away
A sticking caliper can cause rather severe pulsing too. It is worst when applying the brakes but you will often notice it in the steering wheel a little too when not braking. You would probably have noticed the pistons would not push back into the caliper housings easily. I suspect you'll find the cause of the problem before you get to the calipers. GM typically has very little trouble with calipers.
Tuesday, May 25th, 2010 AT 12:22 AM