Brake fluid pressure control apparatus in an anti-lock control system

Claim 1

A brake fluid pressure control apparatus of a type equipped with an electromagnetic valve device for controlling communication between a master cylinder and wheel cylinders through a supply passage and communication between the wheel cylinders and a reservoir through a return passage, and a hydraulic pump for returning the brake fluid in said reservoir to said supply passage; said brake fluid pressure control apparatus comprising: a fluid pressure control valve provided in said supply passage between said master cylinder and said electromagnetic valve device; a stepped piston having a relatively large surface area at one end and a relatively smaller surface area at the other end slidably fitted into said pressure control valve and urged from the side of the small surface area by means of a return spring; a fluid pressure chamber formed between a stepped portion of said stepped piston and a housing of said fluid pressure control valve in such a manner as to communicate with said supply passage; a back pressure chamber formed at said one end of said stepped piston in such a manner as to communicate with said hydraulic pump so that a return-pressure-fluid from said wheel cylinders, at the time of a decrease of the pressure during anti-lock control, is pressurized by means of said hydraulic pump, is introduced into said back pressure chamber to achieve a pressure accumulation therein by moving said stepped piston against the force of said return spring, and is returned to the master cylinder in an amount corresponding to the change in volume of said fluid pressure which change corresponds to the movement of said stepped piston, and further comprising a return fluid passage and a check valve provided in said stepped piston for returning the return-pressure-fluid to the master cylinder, said return fluid passage including a throttle portion.