A filler spout comprising a tubular body having a transverse partition pierced by an opening forming a valve seat which is associated with a main valve member that is connected to a piston extending in a pilot cavity connected to an exhaust orifice and to a pilot duct having a pilot orifice provided with a controlled pilot valve member, the pilot duct opening out to a face of the piston that causes the main valve member to move towards an open position when the pilot duct is fed with a liquid under pressure.