PURPOSE: To provide an isolation system preventing different fluid mediums from leaking around the outer surface of a piston by separating the different fluid mediums from each other.

CONSTITUTION: The invention has a housing 12 provided with an elongated side wall 14 defining a chamber 20, and a pair of end walls 16, 18, and the chamber is divided by a piston 22 into an air compartment 24 and an oil compartment 26. The piston has a first end surface 28, a second end surface 30, and an exterior wall surface 32 extending therebetween. The exterior wall surface separates and seals the air compartment from the oil compartment by means of the housing side wall. A closed annular space provided between the first and second end surfaces of the piston is defined by the housing side wall. Since the annular space is communicated with the outside of the housing by a conduit, a pressure differential is created between the closed annular space and the outside of the housing. Therefore, fluid mediums in the closed annular space flow out to prevent the medium in the air compartment from leaking into the oil compartment.