A reservoir fluidically connected to a fluid pressure source and comprising, a casing, a projection projected from one end of the casing into the casing and fixedly connected with the casing, a cylindrical member sealed hermetically and slidably mounted on the projection, a groove formed between a peripheral surface of the cylindrical member and an end portion of the projection and fluidically connected with the fluid pressure source, a chamber formed between the other end of the casing and an outer peripheral surface of the cylindrical member, and a spring interposed between the other end of the casing and the outer peripheral surface of the cylindrical member so as to constantly urge the cylindrical member, thereby keeping at a certain height the fluid surface within the reservoir regardless of the change of the fluid quantity reserved within an accumulator.
