Adjustable seat post of bicycle

A seat post of a bicycle, comprising:

• a sliding tube including a saddle support at an upper end and a seat secured to an inner surface of a lower portion of the sliding tube;
• a hollow plunger slidably disposed in the sliding tube for dividing inside of the sliding tube into a first chamber filled with hydraulic fluid and a second chamber filled with both hydraulic fluid and pressurized air, the hollow plunger including an opening on a top, the opening communicating with the second chamber, and two opposite apertures formed through the hollow plunger and communicating with the opening and the first chamber respectively;
• and
• a valve stem slidably disposed through the opening of the hollow plunger and including an obstruction on a top, the valve stem being configured to move to cause the obstruction to close the opening of the hollow plunger in a closed position or open the opening of the hollow plunger in an open position;
• wherein:
• the opening of the hollow plunger is provided with a conic surface;
• a diameter of the obstruction is greater than that of the opening of the hollow plunger;
• the obstruction includes a conic surface on a bottom;
• the valve stem is made of a first material having a first hardness and the hollow plunger is made of a second material having a second hardness less than the first hardness;
• and
• the conic surface of the obstruction is configured to complementarily dispose in the conic surface of the top opening of the hollow plunger, thereby forming an airtight seal therebetween, increasing wear resistance of both the obstruction and the hollow plunger, and increasing a useful life of each of the obstruction and the hollow plunger.