Patent ID: 7752843

Claim:
In a working liquid reservoir for a hydraulic circuit comprising: a housing having a first cylinder bore; a piston which is movably fitted into the cylinder bore and dividing the inside of the cylinder bore into a hydraulic chamber and a gas chamber; and a compression spring positioned in the gas chamber and imparts a force to the piston which is directed toward the hydraulic chamber side from the gas chamber side, the working liquid reservoir being capable of storing a working liquid in the hydraulic circuit which is communicated with the inside of the hydraulic chamber, the improvement comprising (A) a second cylinder bore in the housing whose diameter is smaller than the diameter of the first cylinder bore, the second cylinder bore extending coaxially with the first cylinder bore which defines the hydraulic chamber and gas chamber, (B) the piston having a stepped structure including a large-diameter portion fitted into the first cylinder bore and a small-diameter portion which is fitted into the second cylinder bore whereby the hydraulic chamber in the inside of the cylinder bore is formed into a donut shape, wherein two portions of the piston consisting of the large-diameter portion and the small-diameter portion guide the movement of the piston, and wherein the small-diameter portion of the piston includes a plurality of groove passages along the axial direction of the small-diameter portion, and a land portion arranged between adjacent groove passages defining a guide wall for guiding the movement of the piston, and (C) a communication passage formed between an inner peripheral wall of the second cylinder bore and an outer periphery of the small-diameter portion of the piston, whereby a working liquid can reciprocate between the donut-shaped hydraulic chamber and the hydraulic circuit through the communication passage.