Fail safe indicator for skid control system

A vehicular skid control system for relieving the brake pressure when an incipient skid condition is sensed. The system includes a combined check and bleed valve that controls the flow of hydraulic actuating fluid from a master cylinder to the wheel brakes. An expansion chamber is positioned in the fluid circuit between the check and bleed valve and the wheel brakes. An ancillary source of fluid pressure, such as a power steering pump, normally holds the expansion chamber at a minimum volume condition and holds the check and bleed valve open for normal brake operation. When an incipient skid condition is sensed, the expansion chamber is permitted to increase in volume and the combination check and bleed valve is closed. Closure of the check and bleed valve isolates the master cylinder from the wheel brakes and prevents the application of further pressure to the wheel brakes. Expansion of the expansion chamber relieves the existing pressure in the wheel brakes to prevent skidding. A regulator is provided in the circuit connecting the source of ancillary fluid with the expansion chamber for maintaining at least a predetermined pressure in the circuit. The pressure maintained by the regulator is dependent upon the pressure in the brake circuit. A fail safe device is provided that provides an actuating circuit from the master cylinder to the wheel brake in the event of failure of the ancillary pressure source to provide sufficient pressure to hold the check and bleed valve in the normal braking position. An indicator is provided that is responsive to the pressures at the master cylinder and wheel cylinder for indicating if a failure has occurred in the system. Certain of the elements are spring biased in opposition to the bias exerted by the source of ancillary fluid pressure so that these elements may be automatically cycled each time the vehicle engine is started and stopped.