zero position indicated by the arrow 70 the first flange extending approximately 0 08 inch and the latter two extending 0 1 inch around the inner periphery of a circle having a radius of 0 32 inch recessed adjacent thereto and about 3 64 inch inches below flanges 43a are the annular planes 43c which are centered from the reference point at angular clockwise positions of approximately 50 u00b0 190 u00b0 and 330 u00b0 respectively each extending approximately 0 2 inch around the periphery on a circle having a radius of about 0 31 inch near the centers of each of the lugs 47b is a dimple 72 which is respectively designed to accommodate a small bead 71 just above the clockwise end of adjacent ones of the recessed flanges 43c so that when the cylinderical housing 47 is rotated in a counterclockwise direction lugs 47b engage flanges 43c and the beads 71 are accommodated in dimples 72 firmly locking against rotation when the housing 47 is rotated to open position as shown in fig7 b one of the flange 43a on the clockwise side and bead 71a on the counterclockwise side act as stops engaging lugs 47k to maintain housing 47 in proper operating position stop 71a is overridden when spray housing 47 is turned to locking position 3 another change in the embodiment of fig7 a 7b relative to that of fig2 a is in the shape and thickness of the wall of piston 14 and in the relationship between.