Furniture and Furnishings - Office Chair's sink to ground.

Over the years at my workplace we've had many adjustable office chairs shared by different people at all times of the day and night,after a few months they don't stay up at the top and will sink back to the ground.Anyone know why they do this and can they be fixed by an ordinary person?

what brand/style are they?

I don't know the name brand but it has a hydraulic lever to adjust the seat height just under the seat and a lever to rock the backrest forward or back and a couple other knobs which i don't know what they're used for.When you adjust the seat high it will stay there for a bit but slowly sink back to the bottom.Every chair we have had has done this after about 6 months.I always theorized maybe a seal broke like on a shock but I never see oil down by the chamber, beats me why they do it :confused:

If they are pneumatic, and it sounds like they are, then the cylinder must be replaced
Like a strut or shock on a car, you can't really fix it
Not all pneumatic chairs have replacable cylinders
If your chairs have the replacable kind it is usually not that difficult to do
You would need to know the maker and model of the chair and if the replacements cylinders are available