PURPOSE: To easily change the maximum volume of a storage chamber in a reservoir by a base body of same kind by consisting the reservoir of a reservoir forming member, a piston which is fitted thereto in a slidable manner, and a spring therebetween in a reduced manner.

CONSTITUTION: A recessed part to be opened to an outer surface 34b and a passage 881 which is communicated to the suction port of a return pump and opened at the inner end of the recessed part are formed on a base body. A bottomed-hole cylindrical reservoir forming member 110 to be fitted to the recessed part is arranged in the reservoir 191. A storage chamber 91 to be communicated to the passage 881 is demarcated between the passage and the inner end of the recessed part, and a piston 92 to be slidingly fitted to the reservoir forming member 110 is arranged. In addition, a spring 93 to be provided between the piston 92 and the closed end of the reservoir forming member 110 in a reduced manner is arranged. Thus, the maximum volume of the storage chamber 91 can be set to an optimum value according to the vehicle type by preparing a plurality of kinds of reservoir forming members 110 with different shape and dimensions relative to the base body of the same shape.