1. Field of the Invention
The present invention relates to a brake hydraulic pressure control apparatus for a vehicle, such as an antilock brake system (ABS), which includes a pressure-increasing valve interposed in a hydraulic circuit between a master cylinder and a corresponding wheel cylinder, a pressure-reducing valve interposed in a hydraulic circuit between the wheel cylinder and a reservoir, and a hydraulic pump for pumping brake fluid that flows from the wheel cylinder to the reservoir via the pressure-reducing valve and returning the pumped brake fluid to a hydraulic circuit between the master cylinder and the pressure-increasing valve, and which is configured to control the brake hydraulic pressure within the wheel cylinder through control of the pressure-increasing valve and the pressure-reducing valve and to drive the hydraulic pump in accordance with the status of the vehicle.
2. Description of the Related Art
Heretofore, brake hydraulic pressure control apparatuses of such a type are widely known (see, for example, Japanese Patent Application Laid-Open (kokai) No. 7-89423). The disclosed conventional apparatus utilizes a normally-open solenoid valve as the above-described pressure-increasing valve and a normally-closed solenoid valve as the above-described pressure-reducing valve, and performs brake hydraulic pressure control, such as brake hydraulic pressure control by means of an antilock brake system (hereinafter referred to as “ABS control”), by opening and closing the normally-open solenoid valve and the normally-closed solenoid valve, respectively.
Brake fluid consumed as a result of performance of the brake hydraulic pressure control; i.e., brake fluid that flows into the above-described reservoir via the normally-closed solenoid valve, must be returned, without fail, to a hydraulic circuit upstream of the pressure-increasing valve (hereinafter referred to as a “primary circuit”). Accordingly, in general, the conventional apparatus is configured to drive the above-described hydraulic pump over a period between start of the brake hydraulic pressure control and end of the brake hydraulic pressure control and to continue the drive of the hydraulic pump until a predetermined period of time elapses after the end of the brake hydraulic pressure control, while maintaining a state in which the solenoid valves have been brought into their non-excited states (accordingly, the normally-closed solenoid valve is in a closed state).
Further, the conventional apparatus is generally configured in such a manner that immediately after an ignition switch is brought from an OFF state to an ON state, for so-called initial check, the apparatus drives the hydraulic pump over a predetermined period of time, while maintaining a state in which the solenoid valves are in their non-excited states (accordingly, the normally-closed solenoid valve is in a closed state).
At a point in time when drive of the hydraulic pump is ended after completion of the brake hydraulic pressure control or at the time of initial check, a negative pressure may be generated within a hydraulic circuit from the normally-closed solenoid valve to the reservoir and within a hydraulic circuit from the reservoir to the hydraulic pump (the inlet side thereof) (hereinafter, these hydraulic circuits are collectively referred to as a “secondary circuit”).
The negative pressure generated within the secondary circuit is held so long as the normally-closed solenoid valve is maintained in a non-excited state (i.e., a closed state). Accordingly, when the brake hydraulic pressure control or the initial check is performed repeatedly (in particular, when only the initial check is performed repeatedly), the magnitude of the negative pressure increases each time, with the result that air may be generated in the secondary circuit because of cavitation.
If the above-described brake hydraulic pressure control is performed in a state in which air has been generated within the secondary circuit, the air flows into the primary circuit as a result of drive of the hydraulic pump, whereby an operation stroke of a brake operation member (a brake pedal in the case of a four-wheeled vehicle, a brake lever in the case of a two-wheeled vehicle) increases by an amount corresponding to the volume of the generated air. In other words, the conventional apparatus has a drawback in that the operation feel of the brake operation member may deteriorate because of generation of air within the secondary circuit.