The gas flue spring has a cylinder tube (2) closed at one end by a first end member (7) and containing in a second end member (26) a solid piston rod (6) which slides sealingly in a through bore (27) to project into the cylinder. A piston (4) at the inner end of the rod slides sealingly in the cylinder to divide the cylinder into a first chamber (58) remote from the rod and a second chamber (59) on the side of the rod. The second chamber is filled with pressurised working gas. A flow connection (21) links the first cylinder chamber (58) with the outer atmosphere. A gas filling opening (68) in one of the end members opens at one end into the atmosphere and at the other end is in flow connection with the second cylinder chamber (59). When filling the spring the piston rod is first pushed right in and held and the first chamber is connected to a compressed gas source. The gas container is filled with pressurised gas and after filling, the spring is separated from the gas source.
