An air canister for an air cylinder including a valve is provided. The air canister includes an externally threaded head including an axial chamber and a first sealing ring put on a first groove; an internally threaded cap secured to the head; a spring biased sliding rod including an internal first passage, an internal second passage being blocked from the first passage and extending out of the head and the cap, a first port having a first end communicating with the first passage, and a second port having a first end communicating with the second passage; an externally threaded sleeve secured to both the head and the cap and including a second sealing ring put on an internal second groove, a third sealing ring put on an internal third groove, and a fourth sealing ring put on a fourth groove; and a C-ring clamped onto the sliding rod.