That Sinking Feeling

Our old computer chair was replaced about two years ago because the one we had tended to rear back, posing a tilt-over danger.  The new one has a single lever which allows height adjustment, which I like to keep at about elbow level when I am at the computer.  All was well until a few weeks ago when the seat began to  automatically lower when no one was sitting in it.  That was easy enough to remedy: just depress the lever and the seat would rise to the desired position.  Again, all was well until I found that after being at the computer for several hours, I would be peering at the keyboard from chin height.  In stealth fashion, the seat, slowly and without warning or noticing, would sink to the lowest level possible.  I upended the chair, hoping to find a way to fix it, but saw only the website of the manufacturer.  I knew the warranty had expired, after one year, but I emailed Customer Service for advice on how to adjust the seat.  They immediately responded and offered to send me a new gas cartridge.  Gas!  They also sent me instructions as to how to install it;  one of the tools needed is a hammer.  I haven't gotten around to that yet.  And I'm sinking right now.