If you have a metal cut-off saw, and you only like your chair at or consistently above a specific height, you can force this minimum height despite that stupid hydraulic cylinder. This technique also helps once that cylinder breaks down after 12-24 months and you get this constant sinking feeling that puts your knees up under your chin within 15 minutes of any height adjustment.

Measure the outer diameter of the largest part of your chair’s central shaft. Measure the perfect minimum height between the bottom of the chair (where the shaft emerges) and where the shaft ends down at the rollers. For example, if you always like your chair at max height, put it there (without sitting in it) and then measure that distance.

Go to your hardware store and find a piece of metal pipe, min. ½mm in thickness, ideally steel. Copper might be too soft (go at least 1mm), iron piping might be too brittle.

This pipe will have to have an inner diameter slightly larger than the shaft, and be at least as long as the ideal height distance you measured.

If you get a pre-made piece of pipe (sink drain downspout, for example) that is longer than what you need, you will need that metal cut-off saw to tailor that length to the correct amount.

Take the shaft of the chair apart. The vast majority of chairs literally just sit on that hydraulic cylinder, they don’t even click into a locked-in position much less have any fasteners. Although if you have been using it for a while you may need a rubber mallet to convince the seat to separate from that hydraulic cylinder.

Put your metal pipe over the shaft, put the seat back onto the gas cylinder.

Ergo, you now have a height-defined chair that won’t descend beyond a defined point. You still need the cylinder to connect the chair to the bottom half, but that hydraulic cylinder could be completely dead (with a max height pipe), and it wouldn’t matter one bit.