Air tube for a system to lift the flexible roof of the loading space of a freight vehicle by inflating a bellows

System to lift the roof of the with a flexible sheet (5) covered loading space of a freight vehicle by inflating a bellows (6), comprising an air tube extending at short distance along, preferably below, the roof sheet (5), having along its length one or a plurality of air connections to the bellow (6) or assembly of bellows for lifting the roof sheet, which air tube is connected to the air source and/or drain of the system. The air tube is provided by separate length pieces, each loop shaped, e.g. U shaped, both ends of which are spatially connected to the bellow or assembly of bellows, to extend as a serpentine.