A pneumatic braking system for a motor vehicle

The system is characterised by a front axle brake valve unit (80) which includes first and second selector valves (110,111) having a respective first inlet (110a,111a) connectable in operation to a first pressure source (71), a respective second inlet (110b,111b) connected to the outlet of an emergency control valve (100), and an outlet (80b,80c) connected to the brake cylinder (50,51) of a corresponding wheel of the front axle and capable of being positioned selectively in communication with the first or the second inlet of the selector valve (110,111) in dependence on the pressure difference between these inlets (110a,110b;111a,111b). Each selector valve (110,111) is in particular predisposed in such a way that it normally allows communication between its outlet (80b,80c) and its first inlet (110a,111a) as long as the air pressure at the first inlet (110a,111a) is greater than or equal to the pressure at the second inlet (110b,111b). The emergency control valve (100) is formed in such a way that in normal operation it provides to the second inlet (110b,111b) of the selector valves (110,111) an air pressure less than that present at their first inlet (110a;111a).