You might have nothing more than warped front brake rotors. That is common on any vehicle and will cause the brake pedal to bounce up and down. It might also cause a shimmy in the steering wheel.
If the vibration is faster and is heard more than felt, it could be due to the anti-lock brake system activating, assuming your vehicle has anti-lock brakes. The cause for that should be diagnosed starting with an inspection of the front tone rings. GM has a lot of trouble with their anti-lock brake systems anyway, but they are now having a lot of trouble with wheel bearings too. The bearings develop a small amount of play that is perfectly normal, but that is enough to cause a temporary loss of wheel speed signals. Normally that will cause the yellow ABS warning light to turn on when it gets bad enough. If the wear isn't that bad, the system might activate when it isn't needed. That can cause what feels like brakes that are less effective than normal.
If you can't tell the difference between warped rotors and the ABS activating, have a mechanic test-drive it to make that determination. If your vehicle doesn't have anti-lock brakes, warped rotors is about all you have to worry about.
Monday, May 9th, 2011 AT 5:24 AM