I bought a new chair a few months ago, and I regretted it immensely because the gas cylinder that controls its height was not longer able to hold itself if someone sits on the chair. Unfortunately, the vendor offers 90 days warranty only, and the problem started to happen after around 120 days.
This chair wasn't exactly cheap and I think it's ridiculous that it lasted less than 6 months so I contacted the vendor either way and explained my situation but they were irreductible. The best solution they could come up with was to sell me a new cylinder at a discount. That´s not really a good deal because I knew it would be a bit of pain to replace it and also it's very likely that in a few months I will have the same issues with this new cylinder.
However, it was actually much cheaper to buy this cylinder than to buy a whole new chair so I decided to give it a go.
The worst part is disassembling the chair to remove the old cylinder. It's really a job for brute force and I had to use some lubricant and go to town with a hammer. First, I needed to remove that piece that holds the wheels which was not so hard but then came the real challenge, which was disconnecting the cylinder from the chair's seat. The chair is quite sturdy but I was afraid I would break it because I was really hitting hard with that hammer!
I finally was able to remove the old cylinder and then all that was left for me to do was to reassemble the chair with the new one. Fortunately, that is much, much easier and all I needed to do was putting the pieces together, sit on the chair and let my body weight and gravity do the rest.
This cylinder seems to be a little "shorter" than the old one, which leaves the chair at an uncomfortable height compared to my desk so I might have to call those idiots again!
But it will do for now.