The contact owns a 2018 Cadillac XTS. The contact stated while driving 45 MPH and attempting to depress the brake pedal, the vehicle failed to respond as needed. The contact stated that the brakes had been previously replaced; however, the failure persisted. The contact self-diagnosed the vehicle and determined that the rear brakes were failing. The contact stated that a dealer was contacted. The vehicle was taken to a dealer where it was inspected, and the contact was informed that there was no failure found with the rear brakes. The vehicle was not repaired. The manufacturer was notified of the failure but offered no assistance. The contact related the failure to NHTSA Campaign Number: 18V576000 (SERVICE BRAKES, HYDRAULIC); however, the VIN was not included in the recall. The contact was referred to the NHTSA Hotline for assistance. The failure mileage was 51,870.