The contact owns a 2020 Buick Encore GX. The contact stated that while shifted into reverse on several occasions, the brake was depressed and there was an abnormal foghorn sound coming from the driveras side rear wheel. The vehicle was taken to a dealer however, the failure was not duplicated. The vehicle was taken to the residence where the rear driveras side wheel was removed. The caliper was removed, and the contact became aware that piston inside the caliper had fractured in half. The vehicle was not repaired. The manufacturer was notified of the failure. The failure mileage was approximately 101,000.