A relay valve is provided for a vehicle. The valve includes a parking brake piston that is displaceable in a housing and an axially displaceable cuff, a first compressed-air chamber for receiving compressed air from a control line is formed between the parking brake piston and the housing. The valve is also provided with a first spring element, which axially applies a pre-loading force to the cuff, and with a second spring element, which axially applies another pre-loading force to the parking brake piston. An axially displaceable service brake piston is arranged axially between the parking brake piston and the cuff, the service brake piston rests at least radially against the parking brake piston and radially against the housing in a sealing manner, in order to form a second compressed-air chamber.

A control apparatus is configured to control a brake of a unit hauled by a vehicle. The vehicle includes a plurality of brakes for braking the wheels of the vehicle, a trailer brake for braking the hauled unit, and a parking brake for keeping the vehicle braked while it is stationary. The control apparatus includes a line for sending a fluid to the trailer brake through a control valve in order to brake the hauled unit in certain working conditions, a further line for sending a further fluid to the trailer brake in order to activate the trailer brake in other working conditions, and a valve device for controlling flow of the further fluid in the further line, the valve device being responsive to a signal indicative of the status of the parking brake.