Recently bought replacement gas cylinder as my office chair was refusing to lift. For the life of me I can't figure out how to get the old cannister out of the chair. Is it simply brute force (ie hammer)? Don't want to resort to that until I've had advice. I removed the padded chair section from the legs-and-all-other section and can see that the canister is not attached to anything. It's just a very tight fit into the vertical tubular piece between the chair seat and the legs as far as I can see. Any suggestions please? Thanks.