The amplifier has a valve selectively cooperating with two coaxial annular seats to vary differential pressure between two chambers of an enclosure based on axial position of a plunger with respect to a piston. The valve has a bushing (29) in close contact with a widened part of pistons axial bore and external surface of a part of an elastomer unit. An annular support (24) near the surface is formed as a single piece with the bushing.
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