Some owners of 2016 Cadillac XTS vehicles may comment on a hard brake pedal on a cold start and/or a service brake assist message. The technician may find DTC C027B or P050F set in the EBCM. On some vehicles, the connector at the KR14 brake booster pump motor relay was not fully seated during installation. Inspect the connector for proper seating.
After seating the connector, deplete the brake booster vacuum by depressing the brake pedal several times with the engine off, and then restart the engine and confirm that the brake booster pump runs and that there is normal function of the brake pedal.
Example of a fully-seated KR14 connector.