1. IN A LOOM INCLUDING A DROP BOX, A PNEUMATIC WEFT HOLDING DEVICE, COMPARTMENTS IN SAID DROP BOX WHICH OPEN IN THE DIRECTION OF THE LAY, SUCTION PORTS IN THE SIDE WALL OF SAID DROP BOX WHICH ARE ADJACENT SAID COMPARTMENT OPENINGS, A VACUUM DISTRIBUTOR CHAMBER FOR SAID PNEUMATIC WEFT HOLDING DEVICE, SAID VACUUM DISTRIBUTOR CHAMBER HAVING A PLURALITY OF EQUALLY SPACED APART OPENINGS WHICH CONNECT TO A PLURALITY OF CORRESPONDING SUCTION CONDUITS LEADING FROM SAID VACUUM DISTRIBUTOR CHAMBER TO SAID SUCTION PORTS IN SAID DROP BOX, THE IMPROVEMENT WHICH COMPRISES A DISTRIBUTING SLIDE VALVE ARRANGED IN SAID VACUUM DISTRIBUTOR CHAMBER FOR DISPLACEMENT ALONG SAID OPENINGS IN SAID DISTRIBUTOR CHAMBER, SAID SLIDE VALVE HAVING OPENINGS WHICH ARE SPACED FROM ONE ANOTHER AT THE CENTER OF THE VALVE BY THREE TIMES THE DISTANCE BETWEEN ADJACENT OPENINGS IN THE DISTRIBUTOR CHAMBER AND ABOVE AND BELOW THE CENTER OF THE VALVE BY TWICE THE DISTANCE BETWEEN ADJACENT OPENINGS IN THE DISTRIBUTOR CHAMBER TO ADAPT THE SLIDE VALVE TO SELECTIVELY CLOSE SOME OF SAID OPENINGS IN SAID VACUUM DISTRIBUTOR CHAMBER IN RESPONSE TO THE PREVAILING POSITION OF THE DROP BOX RELATIVE TO THE LAY.