is connected to an inlet port 22 on the filter head 16 by means of connecting nipple 24 when the inlet valve 20 is opened pressurized air from an air compressor passes through the pressure regulator 18 and enters the inlet port 22 of the filter head as seen in fig3 the air stream upon entering inlet port 22 is directed downwards as at 26 into chamber 25 which is defined by the space surrounding generally cylindrical porous drying filter 27 and casing 29 of air drying filter 14 at the same time a pressure equalizing tube 28 located in the base plate 30 of the filling chamber 12 and near the inlet port 22 directs a small amount of air upwards as indicated by arrows 23 and above the free flowing balancing material in the filling chamber 12 this equalizes the air pressure in the filling chamber 12 with the air pressure that is passing into the air drying filter 14 the air stream passes through porous filter 27 as at 31 and returns to the filter head 16 via central tube 33 as at 35 and into the outlet 32 of the filter head 16 the outlet 32 of the filter head 16 includes a mixing chamber 37 where the tire balancing material is drawn into the air stream from filling chamber 12 located thereabove as will hereinafter be described a venturi nozzle 34 is positioned in the mixing chamber 37 and compresses the air flow 35 as it passes from the central tube.