An apparatus for controlling passage of a pressure signal, the apparatus including: a first passageway for a first fluid therein; a second passageway for a second fluid therein; a chamber interposed between the first and second passageways (16, 26); a first, movable diaphragm located across a junction of the first passageway and the chamber, the first diaphragm being movable to plural positions; a second, movable diaphragm located across a junction of the second passageway and the chamber, the second diaphragm being movable to plural positions; a third fluid, the first and second diaphragms (40, 50) bounding the third fluid there between and interposed between the first and second passageways at the chamber; and a device for varying a volume of the third fluid bound between the first and second diaphragms and interposed between the first and second passageways at the chamber, a variation of the volume of the third fluid moving the first and second diaphragms; the first and second diaphragms (40, 50) each having at least one position that permits passage of a pressure signal between the first fluid in the first passageway and the second fluid in the second passageway through the third fluid bound between the first and second diaphragms, and the first and second diaphragms (40, 50) each having at least one position corresponding to an increase in volume of the third fluid between the first and second diaphragms (40,50) by the device that prohibits passage of a pressure signal between the first fluid in the first passageway and the second fluid in the second passageway through the third fluid bound between the first and second diaphragms. a first passageway for a first fluid therein; a second passageway for a second fluid therein; a chamber interposed between the first and second passageways (16, 26); a first, movable diaphragm located across a junction of the first passageway and the chamber, the first diaphragm being movable to plural positions; a second, movable diaphragm located across a junction of the second passageway and the chamber, the second diaphragm being movable to plural positions; a third fluid, the first and second diaphragms (40, 50) bounding the third fluid there between and interposed between the first and second passageways at the chamber; and a device for varying a volume of the third fluid bound between the first and second diaphragms and interposed between the first and second passageways at the chamber, a variation of the volume of the third fluid moving the first and second diaphragms; the first and second diaphragms (40, 50) each having at least one position that permits passage of a pressure signal between the first fluid in the first passageway and the second fluid in the second passageway through the third fluid bound between the first and second diaphragms, and the first and second diaphragms (40, 50) each having at least one position corresponding to an increase in volume of the third fluid between the first and second diaphragms (40,50) by the device that prohibits passage of a pressure signal between the first fluid in the first passageway and the second fluid in the second passageway through the third fluid bound between the first and second diaphragms.