Computer chairs are not intended to last forever, and, sooner or later, things will start to go wrong. The most common fault with a computer chair is that it no longer will stay up. The fault lies in the gas cylinder that sits in the upright of the chair beneath the seat. As the chair becomes worn, the seals can become damaged, causing the gas to leak from the cylinder. Once the gas pressure decreases, it becomes hard for the seat to stay up under weight.
First remove the chair's base. Turn the chair upside down and use a screwdriver to lift up the holding clip. The base can then be remove with a bit of encouragement. Turn and pull the base until it slides away from the cylinder. As the base comes away, you will notice the washers and ball bearings. Put these to one side. You should now have four small parts set aside, plus the base and legs of the chair.
Now remove the cylinder itself. The cylinder isn't screwed in, but is a taper fit and can be removed by using a pipe wrench. Place the pipe wrench on the main body of the cylinder and gently twist in either direction. If the cylinder is a little reluctant to come out, then a squirt of a lubricant such as WD-40 will usually do the trick. For very reluctant cylinders, try a few sharp raps and then allow the lubricant to soak in for a while before continuing.
Now that the old cylinder is out, use it to be sure and get the correct replacement, as there are thousands of models of computer chairs on the market, all having different dimensions. The new cylinder is installed by inserting the tapered end into the mechanism by hand.
Replace the ball bearings and washers in this order--largest washer, bearing and smallest washer. Cover with the base and lower the clip.
Turn the chair to the upright position and then sit on it and tighten the tapered joints.
This is a pressurised container, and eye protection should be worn during the replacement.