Office chairs can't last forever. The chair's gas cylinder -- the tube mechanism between the seat and base that controls the height of the chair -- eventually fails; this leaves you with a sinking feeling whenever you use the chair. Two options exist to repair a sinking office chair.
Contact your chair's manufacturer and purchase a replacement gas cylinder. Company contact information is typically found on a label under the seat, on one of the arms or under the gas cylinder attached to the centre of the base.
Turn the chair on its side, and remove the mounting screws holding the original gas cylinder to the seat assembly and base. Remove the base and seat, and discard the old cylinder.
Place the new gas cylinder in the base; reattach any mounting screws. Replace the seat, and reattach any connecting screws. Sit in the chair and move the height-adjustment lever to test the new cylinder.
Use the height adjustment lever to set the seat at the optimum height.
Turn your chair on its side. Measure the length of the upper chamber of the gas cylinder. This is the section thinner in diameter that fits inside the larger, bottom section. Write down this measurement.
Place a 1-foot section of PVC pipe into a vice, tightening it down securely. Draw a line corresponding to the length of the upper section of the gas cylinder. Cut the pipe to the proper length with a hacksaw.
Face the length of the pipe in the vice; cut a straight slit with the hacksaw down its length. Remove the pipe from the vice.
Pry the PVC pipe slightly open, and place it around the top portion of the gas cylinder. Hold the pipe in place after installing it on the cylinder using three, screw-tightened clamps. The seat's height won't adjust, but it won't sink.