20 is mounted in the mounting hole 13 of the adapter 10 extends out from the rear end of the adapter 10 and has a front end a rear end a through hole 21 a first inner thread 25 a second inner thread 26 and at least one air channel 215 and may further have an outer flange 22 and an external thread 23 the through hole 21 is defined longitudinally through the bushing 20 and has an inner surface the first inner thread 25 is formed on the inner surface of the through hole 21 adjacent to the front end of the bushing 20 the second inner thread 26 is formed on the inner surface of the through hole 21 behind the first inner thread 25 the at least one air channel 215 is defined longitudinally in the front end of the bushing 20 and communicates with the through hole 21 the at least one air channel 215 may have a front opening and a rear opening the front opening is defined at the front end of the bushing 20 the rear opening is defined at the inner surface of the through hole 21 the outer flange 22 is formed on and protrudes radially from the front end of the bushing 20 is mounted in the cavity 11 of the adapter 10 and presses against an inside wall of the rear end 12 of the adapter 10 the external thread 23 is formed on the bushing 20 and engages with the internal thread of the mounting hole 13 of the adapter 10 the first valve member 30 is mounted in the through hole 21 through the.