This is kind of an unusual question, but does anyone know how the hydraulic lift mechanism in an office chair works? The one I have at home is constantly (CONSTANTLY) sagging to the lowest position. The rest of the chair is in fine shape, but this lift thing is bugging me to no end. I'd like to try and fix it if possible, but I don't know the first thing about how this works or why it's broken.

Typical failure is in the pneumatic cylinder. Once they leak, they sag like hatchback rear deck struts.

Check the set screw where the handle meets the cylinder, te problem might be to simply back the set screw off a tad. There is a pin at the top of the cylinder when that is pushed down it releases pressure, so make sure that it is not being pressed in anyway.

Hmm. Thanks for the replies, there's no set screw between the handle and the top of the cylinder. I guess that means I'm screwed... no pun intended. Well, I guess I'll see how long I can live with this. Kind of a shame, the rest of the chair is in great shape.

Your prospects depend somewhat on how "disposable" the chair is. When this happens to one of the Steelcase chairs where I work, we contact the outfit that supplies us with the chairs. A guy shows up and replaces the cylinder on site, under warranty (something like a 15 or 20 year warranty, IIRC).