From NHTSA and GM:
GM Recall #: N182167900 NHTSA # AWAITING# Date Issued: Aug 30, 2018 Recall Title:
Trapped Hydrogen Gas Rear Brakes
General Motors has decided that a defect which relates to motor vehicle safety exists in certain 2018–2019 model year GMC Terrain vehicles; 2018 model year Chevrolet Malibu vehicles; 2018–2019 model year Chevrolet Cruze, Equinox, Volt, Impala, and Bolt vehicles; 2018–2019 model year Buick Lacrosse and Regal vehicles; and 2018–2019 model year Cadillac XTS vehicles. In a small number of these vehicles, the rear-brake caliper pistons may contain trapped hydrogen gas that could be released into the vehicle’s brake system.
Safety Risk Description:
If gas is present in the brake system, rear-brake performance may be reduced, increasing the risk of a crash.
Dealers will “bleed” the brake system to remove any gas from the rear brake system free of charge.