Air pressure tube assembly for elevation-adjustable seat stand

An air pressure tube assembly for an elevation-adjustable seat stand, comprised of a barrel, an inner tube mounted inside said barrel, a top cap mounted in one end of said barrel, a first valve and a second valve connected in a line between said top cap and said inner tube inside said barrel, a bottom cap fastened to one end of said inner tube remote from said top cap, a pin adapted to either close or open said first valve and said second valve, a bolt fastened to said top cap and extended out of said barrel, small and big O-rings, and a piston assembly axially movably mounted in said inner tube, wherein:

• said top cap, said first valve, said second valve, said inner tube and parts of said piston are respectively injection-molded from plastics;
• said top cap is shaped like a tapered tube, comprising a bottom flange raised around the periphery of a bottom end thereof, a plurality of longitudinal ribs spaced around the periphery and perpendicularly extended said the bottom flange to a top end thereof, a center through hole extended through the top and bottom end thereof, and a neck in the center through hole adapted to hold said bolt in the center through hole of said top cap;
• said first valve comprises a bottom coupling chamber, a center hole disposed in communication with said bottom coupling chamber and adapted to receive said pin, and a stepped peripheral wall, the stepped peripheral wall of said first valve comprising a front section, which holds one small O-ring, a rear section, which holds one big O-ring, and an intermediate section connected between said front section and said rear section, which holds one big O-ring;
• said second valve comprises a center through hole adapted to receive said pin, a front coupling flange coupled to the bottom coupling chamber of said first valve, a stepped peripheral wall, an air hole transversely extended through a middle part of the stepped peripheral wall of said second valve and disposed in communication with the center through hole of said second valve, and an annular coupling groove on a bottom end of a rear part of the stepped peripheral wall of said second valve;
• said inner tube comprises a transverse partition wall, which blocks one end of said inner tube and holds one small O-ring in contact with said second valve, an annular coupling flange axially forwardly extended from said partition wall and coupled to the annular coupling groove of said second valve, a center through hole extended through said transverse partition wall at the center within the annular coupling flange of said inner tube and adapted to receive said pin, and a plurality of longitudinal ribs spaced around the periphery thereof and disposed in contact with an inside wall of said barrel.