Fix this problem right away as it is a safety issue and it will ruin your brakes as you try to run the car. You will think you have a flat tire because of the rumble coming out of the front end through the steering wheel. If you are aware you can push the brake pedal back up with your foot while driving. Solution is to take the car in right away to have the brake pedal changed. Also you may have ruined your brakes so they should be checked. Most likely your brakes have been burned up. GM is aware of this problem and should issue an immediate safety recall.