I posted a month or so ago in this forum about what I thought was an anti-lock problem. It felt like the anti-lock was kicking in when I hit the brakes lightly at 50+ speeds. Several people thought it could be warped rotors and the classic brake pulsation. Sometimes it gets pretty bad, but seems to quit with heavy braking. There is a loud vibration noise of sort at the same time as you feel it in the peddle.

I've since put new front rotors on with semi-metallic pads and it still did it immediately afterward and seems to have gotten a little worse. Before I replaced the back ones too, I thought I'd take the wheels off, inspect, and retorque. I found some radial marks on the rotors (not a circle around the whole surface) and wondered if that could be an indication of the pads jumping on & off of the surface. Hopefully my image posts good:

http://www.focaltechinc.com/josh/100_3820.jpg

Another question I would like to throw out there, does anybody think that semi-metallic pads could correct a slightly warped rotor, since those pads are abrasive? Seems like it would have the effect of grinding the high spots down. Right now the rear pads are ceramic. Thanks.

Naw! That looks normal to me. I see that on mine. I've always felt it is an indication of where the pads stopped on the rotors the last couple of brakings.

When you replaced the pads did you measure rotor runout on at least the front rotors? If you did not then you need to get back in there and do it, or have it done. If runout is within tolerance, then I'd go after the ABS, although it IS rather difficult to diagnose this from just the symptoms you describe. In your earlier posts did you mention whether or not you get ABS code? Have you tried disconnecting the ABS leads to each of the front and rear wheels, then testing the brakes? That test might tell you if it is an ABS problem or not. If the strange pulsing goes away that would point to the rotors, although that rotor runout test could pinpoint bad rotors for sure. Personally, I don't think pads are the problem.

I put new front rotors (Raybestos PG Plus) and pads on at the same time. A mechanic told me chances are the fronts would be warped if any. I don't have equipment to check runout, is that something that is done with the rotor mounted on the hub? Could new ones be out of tolerance? Just to clarify I haven't replaced anything in the back yet. The photo is of a back rotor.

When I posted before someone did ask if the ABS light comes on. It doesn't, but on other vehicles I've had I'm pretty sure it didn't come on even if the ABS kicks in (snow driving). That'* a good thought pulling the plug on each one to see the effect. I don't want to trigger an ABS code/light which I might need to get somebody to reset though.

Talked to a friend today who has what sounds like the exact same problem on a brand new Impala. Mechanics said they couldn't duplicate it. His does it primarily descending a mountain he drives.

Woody, I see that you said that you carefully torqued the wheels on. That'* good, as uneven torque will cause warping in fairly short order. Did you also clean up the hubs before putting on the new rotors? Any crud that had built up on the hub and not removed will cause the rotors not to lay flat against the hub. Also, the "low traction" lamp should come on with ABS activation, not the ABS light. That should come on only when a problem is detected in the system.

It is hard to tell from your picture, but if those marks on the rear rotor also corresponds to a dark area on it, that could be your problem. In the picture, it looks darker there but it is hard to tell if that is due to lighting.

You may find this section of a TSB helpful....

Brake Pulsation

BRAKE PULSATION

Brake pulsation is caused by brake rotor thickness variation. Brake rotor thickness variation causes the piston in the brake caliper, when applied, to "pump" in and out of the caliper housing. The "pumping" effect is transmitted hydraulically to the brake pedal. Brake pulsation concerns may result from two basic conditions:
1. Thickness Variation Caused by Lateral Run Out (LRO). - LRO on a brake corner assembly is virtually undetectable unless measured. If the brake corner is assembled with excessive LRO (greater than 0.050 mm (0.002 in), thickness variation will develop over time and miles. Excessive LRO will cause the brake pads to wear the brake rotors unevenly, which causes rotor thickness variation. Pulsation that is the result of excessive Lateral Run Out usually develops in 4,800 - 16,000 km (3,000 - 10,000 mi). LRO can be induced when uneven torque is applied to wheel nuts (lug nuts). Improper wheel tightening after tire rotation, spare tire usage, brake inspection, etc. can be the cause of pulsation. Again, it usually takes 4,800 - 16,000 km (3,000 - 10,000 mi) AFTER an event for the condition to surface. The owner or driver does not usually make the connection between the service event and the awareness of the pulsation. The proper usage of torque wrenches and/or torque sticks (torque limiting sockets) will greatly reduce or eliminate the pulsation conditions after wheel service events. The improper use of impact wrenches on wheel nuts greatly increases the likelihood of pulsation after wheel service.

The following are examples of pulsation conditions and reimbursement recommendations:
^ If the customer noticed the condition between 4,800 - 16,000 km (3,000 - 10,000 mi) and it gradually got worse, normally the repair would be covered. The customer may tolerate the condition until it becomes very apparent.

^ If a customer indicated they had wheel service, ask who performed the service. Then;

- If a GM dealer performed the service, consider paying for the repair and then strongly reinforce the use of torque sticks at the dealer. Two common size torque sticks cover 90% of all GM products. Each technician needs to use torque sticks properly every time the wheel nuts are tightened.

- If the customer had the wheel service done outside of a GM dealership, normally GM would not offer any assistance.

2. Thickness Variation Caused by Brake Rotor Corrosion - Rotor corrosion is another form of thickness variation, which can cause a pulsation concern and can be addressed as follows:

^ Cosmetic Corrosion:

In most instances rotor corrosion is cosmetic and refinishing the rotor is unnecessary.
^ Corrosion - Pulsation Caused by Thickness Variation (Lot Rot/Low Miles - 0-321 km (0-200 mi):

At times more extensive corrosion can cause pulsation due to thickness variation. This usually happens when the vehicle is parked for long periods of time in humid type conditions and the braking surface area under the pads corrodes at a different rate compared to the rest of the braking surface area. Cleaning up of braking surfaces can be accomplished by 10 - 15 moderate stops from 56-64 km/h (35 - 40 mph) with cooling time between stops. If multiple moderate braking stops do not correct this condition, follow the "Brake Rotor Clean-Up Procedure" below.
^ Corrosion - Pulsation Caused by Thickness Variation (without rotor flaking/higher mileage - 3,200-8,000 km (2,000-5,000 mi):

In some cases, more extensive corrosion that is not cleaned up by the brake pad over time and miles can cause the same type of pulsation complaint due to thickness variation. In these cases, the rotor surface is usually darker instead of shiny and a brake pad foot print can be seen against the darker surface. This darker surface is usually due to build-up, on the rotor material surface, caused by a combination of corrosion, pad material and heat. To correct this condition, follow the "Brake Rotor Clean-up Procedure" below.
^ Corrosion - Pulsation Caused by Thickness Variation (with rotor flaking/higher mileage - 8,000 + km (5,000 + miles)

At times, more extensive corrosion over time and miles can cause pulsation due to thickness variation (flaking). This flaking is usually a build up, mostly on the rotor material surface, caused by a combination of corrosion, pad material and heat. When rotor measurements are taken, the low areas are usually close to the original rotor thickness (new rotor) measurement and the high areas usually measure more than the original rotor thickness (new rotor) measurement (depending on mileage and normal wear). To correct this condition, follow the "Brake Rotor Clean-up Procedure" described below.
Important: In some flaking instances, cleaning-up this type of corrosion may require more rotor material to be removed then desired. Customer consideration should be taken in these situations and handled on a case by case basis, depending on the amount/percentage of rotor life remaining and the vehicle'* warranty time and miles.

Thanks for the TSB Archon, that'* good information to have. I've been torquing lugnuts ever since reading about rotor warpage. The hubs looked pretty clean before I put the rotor back on, I did look at them for buildup. The Low Traction lamp has never come on, so I can say (assuming the lamp isn't burnt out) its not the ABS kicking in right? I've just read in the Service Manual that runout limit (measured on vehicle) is 0.003" in the front and 0.004" in the rear, however allowable rotor thickness variation is only 0.0003" in the front and 0.0004" in the back. Next I need to invest in a micrometer and Dial indicator.

Just curious if anybody has had problems with rear rotors, or only front?

I would think that at least for the moment, we can ignore the ABS system as the problem. However, when you have the opportunity, try what was suggested about the wheel speed sensor. It will turn on the light while it is disconnected, but will go back out after you reconnect it.

Then when it is reconnected, sometime when you can safely do it, find some wet pavement and apply the brakes firmly enough to where it activates the ABS. See how that feels as compared to what you are experiencing, and check the low traction light.

Is there any play in the front end when you jack up the car and shake the wheel side to side, perhaps like a loose bearing?

It is possible to get new rotots that are slightly warped. GM has a TSB on using shims under the rotor to correct that problem.

I had to replace the rear rotors, as the passenger rear was sligtly warped when I bought the car.

Unfortunately, 0.0003-4" is almost impossible to measure. That'* how perfect the rotor thickness is supposed to be.

I've had cars that I've never been able to fix for pulsing peddle. I never bother to get the rotors turned; either reuse them as is, or get new rotors. Clean the hubs as well as you can.

And finally.... keep your fingers crossed.

Some good news, after retorquing the rear wheels, the pulsation seems to have noticeably improved. I think they had been zipped on with an air gun last. The Bonneville is new to me (May of this year). I've used an impact (inspite of better advice) on my S10 and Blazer wheels for 8 years without any such problems.

Along with ruling out the anti-lock (in my mind) by disconnecting the sensors, I am thinking about replacing my rear ceramic pads with semi-metallic pads, to see if the abrasiveness will true up any thickness variation for me. Hate to wash my wheels all the time, but I'd sure like the brakes to be smooth

Not that I can feel.