1. Field of the Invention
The present invention relates to a vehicular brake apparatus having a master cylinder, a hydraulic pressure source, and a hydraulic pressure booster.
2. Description of Related Art
There is a vehicular brake apparatus having:
a master cylinder including: a casing; and a master piston of which rear face faces a boosted hydraulic pressure application chamber and which is slidably accommodated in the casing;
a hydraulic pressure source; and
a hydraulic pressure booster including:                a backup piston which is slidably fitted into the casing while a front face thereof facing the boosted hydraulic pressure application chamber and a retraction limit thereof is restricted; and        a pressure regulator built into the backup piston and adjusting the hydraulic pressure output from the hydraulic pressure source and applying the hydraulic pressure to the boosted hydraulic pressure application chamber. In this vehicular brake apparatus, the backup piston moves forward to directly push the master piston against the hydraulic pressure force due to the hydraulic pressure output from the hydraulic pressure source when the hydraulic pressure of the boosted hydraulic pressure application chamber is low.        
In this kind of vehicular brake apparatus, when the hydraulic pressure of the boosted hydraulic pressure application chamber is low, the backup piston is moved forward by moving a control piston forward against the hydraulic pressure force due to the hydraulic pressure output from the hydraulic pressure source so that the backup piston directly pushes the master piston, whereby an effective brake hydraulic pressure can be output from the master cylinder. When the hydraulic pressure of the boosted hydraulic pressure application chamber has lowered due to abnormality in the hydraulic pressure source, since the hydraulic pressure force due to the hydraulic pressure output from the hydraulic pressure and applied to the control piston is small, the operation force required to move the backup piston forward and to directly push the master piston can be made relatively small.
However, when the hydraulic pressure of the boosted hydraulic pressure application chamber has lowered due to abnormality in the pressure regulator while the hydraulic pressure source operates normally, it is necessary to move the control piston forward against the hydraulic pressure force due to the hydraulic pressure output from the hydraulic pressure source and to further move the backup piston forward while directly pushing the master pistols. As a result, a large braking operation force is required to output an effective brake hydraulic pressure from the master cylinder.
To solve this problem, Japanese Patent Unexamined Publication JP-A-2006-240542 proposes a vehicular brake apparatus. In this vehicular brake apparatus, an electromagnetic valve other than the pressure regulator of the hydraulic pressure booster is provided in the hydraulic pressure passage for establishing connection between the hydraulic pressure source and the boosted hydraulic pressure application chamber When any abnormality has occurred in the pressure regulator, the electromagnetic valve is opened to transmit the hydraulic pressure output from the hydraulic pressure source to the boosted hydraulic pressure application chamber, whereby an effective brake hydraulic pressure can be output from the master cylinder by applying a relatively small braking operation force even when the pressure regulator is abnormal.
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