face of screw joint portion 151 prevents air from entering the through hole 15 when the air adjustor button 31 is tightly engaged to the screw joint portion 151 of the bend body when air adjustor knob 31 is unlocked air is permitted to flow inside the through hole 15 and through the longitudinal grooves 152 at the outer edge of the screw joint portion the convex ring 310 and the crevice at the upper end face of the screw joint portion air conduit 32 is secured to the setting member 153 of bend body 10 to communication with the through hole 15 for forming an air passage the inverted l type seal cover 40 is provided with a rail channel 41 along its outer edge which it engages the rail channel 41 receives the rail of groove 16 on the notch provided on the lower end of bend body 10 the engagement of the rail channel 41 and the rail groove 16 allows the l type seal cover 40 to be securely affixed to the bend body 10 the upper end of the seal cover is provided with an opening 42 to communicate with through opening 15 of the bend body 10 and receives the lower end of air conduit 32 the mounting of air conduit 32 and the opening 42 not only enables the inverted l type seal cover 40 and the bend body to be more securely affixed but also enables the air conduit 32 to be more securely retained in opening 42 the lower end of the seal cover 40 is provided with a through opening 43 disposed opposite the cover opening 13 of through opening.