During landing, what should be done when the RED CAS "BRAKE BOTH SYSTEMS FAIL" alert appears?

When the "BRAKE BOTH SYSTEMS FAIL" alert appears during landing, the recommended action is to gently pull the parking brake handle. This maneuver is crucial because it provides an additional means of decelerating the aircraft in the absence of normal braking systems. The parking brake is designed to create drag and assist in stopping the aircraft, particularly in an emergency situation where both primary braking systems have failed.

In contrast, activating the emergency hydraulic system typically would not address the immediate braking failure, as this system is meant to provide redundant hydraulic power rather than directly engage the brakes. Engaging the autobrake system is not effective since it relies on the aircraft's main braking systems, which are already compromised. Decreasing airspeed immediately is a general good practice during landing but does not specifically address the brake failure issue at hand; without brakes, control of the aircraft while on the ground becomes significantly more challenging.

Thus, pulling the parking brake handle is the best response to effectively and safely manage the situation when faced with the alert regarding brake system failure.