DUAL PILOT CHECK VALVE VEHICLE BRAKING SYSTEM

A vehicle braking system is disclosed which includes disc brakes having both service actuators which are operated by fluid pressure, and fail-safe spring actuators which are held off by fluid pressure to permit normal actuation of the service brakes. The braking system includes a first fluid pressure circuit for actuating some of the service actuators and a second fluid pressure circuit for actuating the remaining actuators. Hold-off pressure may be supplied to the spring actuators from either or both of the fluid pressure circuits. A valve is provided which maintains the hold-off pressure in the spring brakes even though the pressure in both of the fluid pressure sources may decrease. However, when the sum of the fluid pressures in both the fluid pressure sources drops below a predetermined amount, the valve vents the hold-off pressure from the spring actuators to permit the latter to effect an emergency brake application.