the forward movement of piston rod 24 within the pneumatic cylinder 23 the passage of air inside the control element 78 is prevented and the presser foot 16 cannot be lifted until the piston 24 returns to its prime position in the new position taken by exchange valve 67 air in the pneumatic cylinders 23 and 52 is released and the force on the actuator through duct 83 stops however valve 75 does not change since valve 63 is still in a different position because the operator continues to engage the pedal 64 the air from exchange valve 67 passes directly into the pneumatic cylinders 23 and 52 through duct 82 moving pistons thereof to their 34 off 34 position in particular the opening of the air passage inside the control element 78 follows the movement of the piston 24 so that the presser foot 16 is caused to lift from the work below with reference to fig4 and 6 the movable thread puller hook 28 moves between the leaf type spring 44 and the projecting part 46 in line with and to a position beyond the cover thread which becomes subtended between the thread guide slot 49 and the needle opening 88 in the sole 47 obviously on the side of the needle eye the cover thread 19 becomes sewn into the row of stitches 89 formed in the workpiece 90 shown in fig4 and 5 during its return stroke the movable thread puller hook 28 draws the cover thread beneath the leaf type spring 44 which clamps it.