in the back region of back portion 112 of casing 108 see fig2 and 6 holes 126 are offset from the longitudinal axis of central passageway 122 and are located beyond o ring gland 118 inner surface 128 of the end of back portion 112 of casing 108 is bevelled drain channel 130 is located circumferentially in the outer surface of back portion 112 of casing 108 inwardly of o ring gland 118 two drain holes 132 are located diametrically opposite of each other in the bottom wall of drain channel 130 this 180 u00b0 location to each other is preferred also the use of three or four equally spaced drain holes 132 is preferred drain holes 132 provide communication between drain channel 130 and central passageway 122 the preferred embodiment of transition portion 134 between front portion 110 and back portion 112 of casing 108 is best seen in fig8 transition portion 134 is of slanted shape between front portion 110 and back portion 112 the inside of transition portion 134 has slanted portion 136 which extends to about the end of back portion 112 of casing 108 the inside of transition portion 134 continues with vertical wall 138 which.