A tube defines a cavity. Normally-open fitting (2) and normally-closed fitting (3) are respectively fitted into the tube to form ports (34),(35) having seats (40),(13). A body secured to fitting (3) defines a chamber (26) to which port (35) leads. Body (4) defines holes (27),(30) leading into chamber (26). A solenoid assembly (6) is slidable within chamber (26), includes a coil, segregates chamber (26) to chambers (7),(29) which communicate respectively with holes (27),(30) and defines a chamber (16), a seat (18), a seal (14) and an orifice (33) leading between seat (18) and seat (13). A rod-piston assembly includes a rod (37) attached to: assembly (6), sealably passing through body (4) into cavity and preventing gas communication between cavity and chamber (7); and a piston with a seal (39). A magnetic rod (15) slides within chamber (16). Spring (17) urges rod (15) towards seat (18) to seal orifice (33). Spring (12) urges assembly (6) towards seat (13) to seal port (35). The coil moves rod (15) and assembly (6). Assembly (6) and the rod-piston assembly are movable between positions wherein: the piston seals against seat (40); and wherein seal (14) seals against seat (13).