This invention relates to a hydraulic brake system for vehicles, and more particularly to a hydraulic brake system for vehicles which are provided with an anti-lock control apparatus.
In a normal brake system for vehicles, a brake fluid pressure control apparatus is used for supplying a brake fluid pressure in response to an actuation of a brake pedal whereby a specific brake fluid pressure is supplied to a wheel cylinder to operate a brake associated with each wheel.
Recently, a hydraulic booster which can boost and drive a master cylinder in response to operation of a brake pedal as a boost source of an output power fluid pressure of a power fluid pressure source has been utilized as a server boost apparatus, and it was proposed to use the output fluid pressure of the hydraulic booster for an anti-lock control apparatus for preventing locking of a wheel upon braking. Such an apparatus has been disclosed in Japanese Patent Publication No. 56(1981)-10219 wherein an auxiliary pressure source, namely, the power fluid pressure source is used, so that a means is provided for blocking a wheel cylinder and a pressure chamber during the antiskid actuation so as to not consume the fluid pressure within the pressure charge of the master cylinder, if a motor or a pump is damaged. Further, any trouble caused by the damage of the pump etc. can be sensed by determining whether the pressure within an accumulator is below a predetermined value or not by use of a pressure sensing switch.
Further, in Japanese Patent Laid Open Publication No. 59(1984)-145655, an apparatus is disclosed in which a fluid pressure and a fluid level are sensed and the antiskid control is stopped. That is to say, the pressure decrease and the decrease of the fluid level are presented as an output of a NOR gate, and a solenoid valve is not actuated after a time determined by a delay element has passed.
In the former prior art, it is feared that the fluid of the reservoir, namely, the remaining quantity of brake fluid will be insufficient upon the detection of the pressure decrease and that the pressure will not be sufficient as a control input.
Also, in the latter prior art, it is desirably disclosed that the fluid level is a control input as is the pressure and the remaining quantity of the fluid to be maintained upon the stop of the anti-lock control, and these are selectively chosen as inputs. Accordingly, when the anti-lock control during the actuation thereof is stopped by the decrease of fluid, a driver is obliged to return to normal brake actuation without any advance warning. In order to prevent this problem, it is required to separately provide a fluid level detector which has a different detecting level for the warning and matching with the stop of the anti-lock control becomes necessary.