Abstract: A sealing device between a reciprocator and a stationary encircling housing comprises first and second rolling diaphragms at opposite ends of the reciprocator and exposed to higher and lower pressures, respectively, diaphragm-supporting liquid in first and second chambers in the respective diaphragms, and pumping means which, owing to the reciprocation, pumps liquid from the second chamber to the first. This means comprises an annular seal carried by the reciprocator, sliding on the housing and acting as a piston and as an inlet non-return valve, and another annular seal carried by the housing, sliding on the reciprocator and acting as an outlet non-return valve.