The unit has a reversible electro-step motor (41), with remote control through electrical pulses. A step rotation is given to a group of drum control cams, initially from a start position into an outwards direction and then give a reverse rotation after a given number of outwards rotations in the forwards direction. The group of control cams has a cam (42) with an increasing rising contour, pref. in steps, to control the movement of the retraction lever (30) from its rest position into each of the following contour steps. The forward steps of the control drum cams, in the outwards direction, starts from the opening position with an increasing action. The valve to deliver a compressed air stream into the preparation sections (28) is activated by a control cam from the group of drum cams. The cam has an active path and an inactive path (54a) round it so that the valve is activated only by the active path and opened when the group of control drum cams rotates in the outwards direction, and the valve cannot be operated when they rotate in the opposite direction.
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