A system for controlling braking of a motor vehicle during downhill driving, in which the vehicle is provided with a service frictional braking device (18) and an auxiliary braking device (25), comprising a retarder brake (26) and a lever (40) for activating the retarder brake by the driver of the vehicle, comprises means (29) adapted to, upon activation of said retarder brake (26) by actuating said lever (40) by the driver for applying a braking action to the vehicle when driving downhill, based upon values of parameters of the driving conditions of the vehicle, calculate whether the auxiliary braking device is able to alone obtain said braking action to be applied to the vehicle. The braking control means (28) is adapted to activate the service frictional braking device to assist said auxiliary braking device when said calculation states that the auxiliary braking device is not able to alone obtain said braking action to be applied to the vehicle. (Fig 1).