Excess gas –– we’ve all had it, no-one likes it, and the same holds true for your braking system. GM says it needs to bleed the brake systems of 230,000 cars because the vehicles have rear brake caliper pistons that have hydrogen gas trapped inside that could be released into the brake systems. ZF, the manufacturer of the brake pistons, didn't properly temper and chrome-coat the pistons, causing hydrogen gas to remain trapped in the bodies of the pistons.
Remember when our parents threatened to stop the car right now in the middle of the highway if we didn't simmer down in the back seat? Yeah, the driver assistance system in the 2014 Chevy Impala is kind of like that, but actually acts on it. A 2014 Impala owner is a bit shaken after his car came to multiple screeching halts without ever touching the brakes.