The brakes are applied automatically at the time of any system failure. The parking brake is operated by the parking/emergency switch on the dash panel or the ignition switch, without physical effort as found in the conventional hydraulic brake systems. Full-power parking/emergency brakes and fail-safe concepts have been accomplished in the straight-air brake systems. However, duplication of these systems to hydraulic brake systems incurred some difficulties due to the physical properties of fluid, and it is generally concluded that the parking/emergency brakes must be mechanical brakes, and the hydraulic pressure should not be maintained for extended periods of time. The electro-mechanical method employing a detent system is a breakthrough in restraining spring force during normal operation.