In many situations, people throw out good office chairs just because of the pneumatic cylinder. Or the wheels are not working. But this is not the solution. If your seat keeps sinking when you sit on it, it only means the pneumatic cylinder is not working.
Many adjustable office chairs use a pneumatic cylinder for lowering and raising the chair height. If any adjustment problems occur, do not throw your chair away. Try fixing it, and you just need a screw and some skills.