not pass to conduit 76 whereas each time probe 32 meets the flat part 90 of said element 33 valve 31 enables the connection between the two conduits 54 and 76 so that pressure reaches the impulse shaper 49 which will send a pressure impulse to input 69 of the pneumatically controlled valve 46 this impulse controls the connection of conduit 67 with the discharge 72 thus causing the discharge of the pilot pressure applied to input 65 of valve 61 since the impulse coming from the shaper 49 is of limited duration when it ceases valve 46 restores the connection between conduit 67 and conduit 57 so that a new loading cycle of timer 47 begins probe 32 of valve 31 in the meantime abandons the flat part 90 and valve 31 is once again closed during the sealing of one box several cycles take place in which air is charged and discharged under pressure at the pilot input of timer 47 if the tape breaks accidentally or if the free end of the tape is not properly positioned on the roller 15 for applying the tape at the start of the sealing cycle roller 12 no longer rotates and thus stops the cam shaped element 33 with probe 32 on the round part or on the flat portion 90 of the cam shaped element in the first case the connection between conduit 54 and conduit 76 is interrupted so that the pressure upstream from valve 31 does not reach the impulse shaper 49 while if a case is.