I though mine needed re-gassing, which  it what brought me to this site!  On inspecting my chair to look for the cylinder,  I turned it upside down looked at the mechanism (nothing more)  pushed and released the side lever, pulled the seat up to full extension and turned it right way up again.   Bingo -seat stayed up, bounce is back.......  Don't know for how long, but it was very firmly deflated before!!
                 jillygee
                
 March 2011