hi,
i had the same problem with my office chair for my home office.
what i did was this a cheap east fix ...
1, put chair on highest level and measure the ram from the top of cylinder to base of seat fixing (measuring the silver piston showing)
2, turn chair over on to front and remove the small clip off the bottom of the ram below the wheeled section.
3, remove the sections piece by piece taking care not to loose any of the washers and bearings etc from inside until your left with basically the piston itself.
4, then get some plumbing white plastic pipe (under sink or waste pipe) and cut to the length that you measured at start.
5, then slide over the ram to just under where it joins the seat section of the chair.
6, then re assemble in reverse order piece by piece ending with the clip on the bottom below the wheel on end of ram.
then your chair will spin and tilt etc but it will not got up and down. a cheap easy fix for about 5 mins work :0)
hope it helps you !! worked for me :0)
martin, March 2010