rings 56 which flank the air inlet 54 an airhorn 58 made of stainless steel tubing and having a 90 u00b0 elbow is bolted to the rear wall 46 so that the airhorn inlet 60 is aligned with the piston top air inlet 54 and the airhorn outlet 62 is located in the center of the piston facing front wall 44 use of the airhorn 58 results in a uniform gust of air against the front wall 44 the front larger diameter portion 50 contains behind the piston screen like front wall 44 a single groove accommodated o ring 56 the piston 42 has a smaller diameter cylindrical side wall 64 which extends between and connects the front and rear larger diameter piston portions 50 52 this smaller diameter side wall 64 has a forward and aft bottom flow slot 65 into the interior of the piston 42 the piston screen like front wall 44 is preferably a sintered stainless steel screen and preferably capable of filtering particles five microns or larger it is mounted within a ring like frame 66 of a slightly smaller diameter than that of the piston 42 the frame 66 fitting slidably through a slot 78 and into a groove 80 located within the piston front larger diameter portion 50 forward of the o ring 56 the travel of piston 42 is limited by a standpipe 67 which extends from bottom outflow port 32 through slot 65 further the standpipe 67 prevents piston 42 from rotating within cylinder 16 perforations 68 in the standpipe 67 facilitate draining of the liquid from the piston 42 the standpipe 67 may be held in place by a bayonet fitting 70 with a rod 71 welded to outflow port 32 in the preferred embodiment the forward.