The present invention relates to a vacuum booster for operating a brake master cylinder in an automobile, motorcycle or the like in a boost mode, and more particularly to an improvement of a vacuum booster in which a holding tube which holds a valve member of a control valve is fitted in a synthetic resin cylindrical valve case with the front end portion coupled to a booster piston and with the rear end opened, and the base end portion of a valve member is fitted in an annular holding groove formed on the outer surface of the front end portion of the holding tube in such a manner that the base end portion is sealingly engaged with the inner surface of the cylindrical valve case. Further, the present invention relates to a holding tube mounting structure in the vacuum booster with which the holding tube is secured to the cylindrical valve case.
In a conventional vacuum booster, as shown, for instance, in Unexamined Japanese Utility Model Publication (OPI) No. 73459/1989, the holding tube is secured to the cylindrical valve case as follows. The front end of the holding tube is abutted against an annular step formed on the inner surface of the cylindrical valve case. Under this condition, the rear end of the holding tube is held with a circlip which is engaged with an annular engaging groove formed on the inner surface of the cylindrical valve case.
The holding tube is combined with the cylindrical valve case in the above-described manner. Hence, when the holding tube is inserted into the cylindrical valve case under the condition that the base end portion of the valve member is held in the holding groove, the base end portion of the valve member is damaged by the annular step formed on the inner surface of the cylindrical valve case. Therefore, the holding tube and the cylindrical valve case must be assembled carefully. Since the annular engaging groove on the inner surface of the cylindrical valve case is difficult to form with the metal mold, the annular engaging groove is formed by machining. This fact increases the manufacturing cost of the vacuum booster together with the fact that it is necessary to use the circlip to hold the rear end of the holding tube.
In another conventional vacuum booster, as disclosed, for instance, in Examined Japanese Utility Model Publication No. 12852/1992, in order to fix a holding tube inserted into a cylindrical valve case, a circlip retaining a rear end of the holding tube is engaged with an annular groove formed in an inner surface of the cylindrical valve case. That is, the holding tube cannot be fixed without the circlip. In order to engage the circlip with the annular groove, a special tool must be used, which makes it rather difficult to assemble the holding tube and the cylindrical valve case with high efficiency.