I did a forum search but didn't get any hits. My son's 06 Cobalt has an intermittent hissing sound under the dash when he hits the brakes( seems worse in warmer weather.) It also seems to be keeping the brakes applied after he releases the pedal, he has to pull back up on the pedal after breaking ir the brake lights stay on. I'm guessing it's probably the brake booster itself, but I don't have a diagram of the brake vacuum circuit to see if there is any type of valve in the system that could be causing the booster to malfunction. Any thoughts? Also, is it as much of a bite in the balls to change as it looks to be?