PURPOSE: To reduce a boosting speed when a wheel brake pressure is low and to increase the boosting speed when it is high by varying an energizing force, exerted on a plunger through the force of a return spring, according to a wheel brake pressure and changing a differential pressure between the front and the rear of a fixed orifice formed to a plunger.
CONSTITUTION: The energizing force of a return spring 60 is exerted on the end part on the small part 62a of a stepped piston 62. Meanwhile, the liquid pressure of a wheel brake flowing through a working liquid passage 64 is exerted on a large part 62b. A set length L of the return spring 60 is changed according to the pressure of a wheel brake. When, during reboosting of an antilock, a wheel brake pressure is low, a differential pressure between the front and the rear of a fixed orifice 53 is low, an amount of working liquid fed to a wheel brake 33 is also low, and slow boosting is carried out. Meanwhile, when a wheel brake pressure is increased, a stepped piston 60 is moved and a differential pressure is increased, a flow rate of working liquid is increased, and rapid boosting is effected.
TANAKA HIROHISA
OSUGA TSUYOSHI
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