Office chairs use a pneumatic gas cylinder to hold the chair at the height you desire. When the chair will not hold at the correct position, you will need to replace the pneumatic lift. Removing the pneumatic lift is not difficult but it will be a dirty task because the outside of the pneumatic gas cylinder has a coat of surface petroleum. You can purchase replacement cylinders from an office furniture store or online through the manufacturer of your chair.
Lay a sheet or some newspaper on the floor large enough for the chair. This will prevent grease from soiling the carpeting on your floor.
Flip the office chair upside down, so the top of the back rest and the front of the seat are sitting on the floor. Locate the clip that is in the centre stem of the chair base bottom.
Pull the clip off the chair base bottom stem with a flathead screwdriver. Pry the clip off with the screwdriver and set it aside. Pull the flat washer off the stem and set the washer aside.
Place a pipe wrench on the end of the gas cylinder and gently twist in either direction to disengage the cylinder. The gas cylinder has a taper on the end that sits in a taper hole in the bottom of the chair. The twisting motion will help free the top of the gas cylinder from the chair.
Pull the gas cylinder straight out of the chair stem. It is a good idea to use a rag when pulling the gas cylinder out because the cylinder will have some grease on the outside.
Place the new pneumatic gas cylinder into the chair stem with the taper rod toward the chair bottom. Secure the cylinder to the chair with the washer and retaining clip.
Set the chair upright and sit in the chair to engage the taper end into the chair bottom. Raise the chair with the lever to the appropriate position.
Some chairs may require the removal of the base. There is a screw or nut securing the base to the chair cylinder. Always follow the manufacturer's instructions that come with new cylinders.