Abstract: The present invention relates to an arrangement in vehicle hydraulic service brake systems for activating at least one of two conventional braking cirucits included in the brake system when an obstacle is detected, by means of a sensor, in the direction in which the vehicle is travelling. The novel matter of the invention resides in that a pressure accumulator is connected to the brake circuit via a valve unit which is adapted, in normal position, to establish free communication between the vehicle main cylinder and the vehicle wheel brakes, and to permit pressure build-up of the pressure accumulator upon depression of the vehicle brake pedal. The sensor is adapted, upon actuation, to activate the valve unit, whereby the communication between the vehicle wheel brakes and the vehicle main cylinder is interrupted, and the pressure accumulator is connected with the wheel brakes for application thereof.