i put new brake pads on the back of my 2008 chevy cobalt sport and now i have a wheel bounce when i put on the brakes hard or quick

You can't do things halfway on cars. Better check out the calipers as well. It appears that you are doing all this work yourself? Are you qualified to do that? Driving a car with bad brakes is not only dangerous to you, but, everyone else on the road and it's illegal. You have to fix this problem immediately or you may wind up in trouble. See this discussion, and, not to mention that the Chevrolet Cobalt is an unsafe car anyway, https://answers.yahoo.com/question/index? qid=20080318120348AAZZ2SG

thanks for your input mark if you are thinking about the switch issue it was taking care of.

Mark, you're brimming with unhelpful pretend information, half truths and alarmist crap. Replacing pads only is not doing things half way. Very often that is all that's required. It's only a job half done if there are other things that need doing. I agree that it would be good to check calipers too, but to suggest that a bad caliper could somehow make a car vibrate on braking is just silly. A warped rotor would make a bit more sense although it seems odd that it would suddenly become noticeable after replacing the pads. I'm wondering it the vibration is actually coming from the back wheels. The sort of vibration that's being described often originates in the front end and is a sign of a worn out axle shaft. By the way it is not illegal in most, if any jurisdictions to perform repairs on your own vehicle. What may be illegal is to charge to perform work on the cars of others, but that's another issue entirely. And as for Cobalts being unsafe, that is pure nonsense. There are hundreds of thousands of Cobalts out there and they are as safe as any other car on the road. People often drive these inexpensive cars because they can't afford anything else. We do them a disservice here when we make false statements that cause them to feel unsafe and force them to spend money they don't have to buy something else.

I never said anything about it being illegal to work on your own vehicle. I merely pointed out that it's illegal to drive any vehicle in all 50 states with faulty brakes. If someone does not know how to repair their car correctly, they need to take it to someone who does, that's all. As for the Chevrolet Cobalt, there are many documented stories about the safety hazards of these cars. Think Chevrolet Corvair. Faulty ignition switches that caused accidents, 303 people died, those are facts. Why do think there was a federal investigation about this. Look, I'm just saying with all of the known problems with this particular vehicle, it's imperative to check out every part of a braking issue before going back on the road. That's just due diligence and good common sense. https://www.cars.com/articles/2014/03/gm-ignition-switch-recall-what- owners-need-to-know/

If you are looking for a cheaper car, at least the Chevrolet Spark got a better review.