Don’t discard broken pneumatic chairs – Fix them for $2 in piping
I fixed mine and it was easy. I sit with my legs scrunched up underneath me so my full body weight (about 170lbs) is acting on the chair at all times. This causes stress to the pneumatic tubing and the plastic that holds it together. At 8 or so hours a day, leaks invariably form within 1-2 years of my using the chair, and I would get a new one.
But that's crazy, especially if there's nothing else wrong with the chair and I'd rather it were in a constant raised position anyway. Desks seem to be a pretty standard height, so I'm not sure why computer chairs are always adjustable. That it's such a widespread feature is strange at least, but it probably sells more chairs in the long run when the shitty feature breaks and you sink back down to kindergarten cubby height.
Turn your chair over resting it on its front with the weight on the chair back. The legs/wheels will be sticking up in the air at an angle. If they aren't already, push the telescoping plastic segments toward the chair seat. Measure the space between the end of these segments and the base of the chair.
It's usually 4.5 inches. Even if it's a little more, it's a good idea to assume 4.5 so you get a little cushion/wiggle room. You don't have to measure the diameter of the tubing, because that's pretty standard at 1.5 inches.
Go to a hardware store and buy a 1.5 inch diameter 4.5 inch long (or whatever your measurement was) piece of PVC pipe. At mine I had to buy a foot of the pipe for $1.99, and they cut it on the premises at no extra charge. Depending on the place there might be a small surcharge for cutting it to spec, though. They even gave me the remaining 7.5" of pipe.
On the chair, in the same position as before, making sure that the chair is raised as high as it will go, look at the bottom end of the center support. There will be a metal clip, which can be pushed on and off with a flat screwdriver. Remove it, and then slide off the base of the chair and set it aside. Remove any washers that come off in the process, and remember where they go. They will be oily, as will part of the tubing. You don't have to wipe the grease away, but mind where you set things down and what you touch afterward.
Insert the cut PVC pipe onto the support. It will drop naturally into the 'raised' telescoping sections. You might have to remove the largest/outer of these to get the pipe in; you can replace it after the pipe is installed if you don't want it visible.
Put the chair back together, including the metal clip at the bottom, and you're done. If you did it right, the chair will not lower anymore, either with the lever or by sitting in it. It's stuck in the highest position and ready to use.