The contact owns a 2018 GMC Sierra 1500. The contact stated that while driving at various speeds, while depressing the brake pedal to slow down, the brake pedal lost pressure, and the vehicle experienced a reduction in braking power. The contact had to depress the brake pedal with additional force to properly slow the vehicle. There was no warning light illuminated; however, the message ‘Brake Assist’ was displayed on the instrument panel. The local dealer was not contacted, and the vehicle was not diagnosed or repaired. The manufacturer was not notified of the failure. The approximate failure mileage was 135,000.