1. Field of the Invention
This invention relates to a brake control system which is capable of adjusting a braking force to be applied to automotive vehicle wheels irrespective of manual braking operation.
2. Description of the Related Art
One typical brake control system is disclosed in Japanese Patent Application Laid-open Publication No. 62-149,543, which includes a variable pressure regulator and a pressure modulator disposed between a master cylinder and a wheel cylinder. The pressure regulator receives a working fluid pressure built-up in the master cylinder and also an external fluid pressure for boosting the master cylinder pressure at a desired boosting rate. The pressure modulator receives the boosted fluid pressure to generate a modulated braking pressure to be supplied to the wheel cylinder as the sum of the boosted fluid pressure and the master cylinder pressure. An external pressure source serves to apply the external fluid pressure as a braking force while brake is not manually operated, for performing traction control operation wherein slippage of driving wheels is detected and suppressed or adjusted at an optimum level. On the other hand, the fluid pressure in the wheel cylinder cannot be lowered below the master cylinder pressure, so that it is not readily possible, during swift braking or braking on a slippery road, to perform anti-locking or anti-skid brake control operation for adjusting the braking pressure at the wheel cylinder so as to maintain the wheel slippage at an optimum level.
Another brake control system is disclosed in SAE Technical Paper No. 84 0468, page 3, FIG. 3, which includes a ball valve and pressure reducing unit both which are disposed between the master cylinder and the wheel cylinder and operated synchronously with each other, as well as an external fluid pressure source for controlling the pressure reducing unit by supplying thereto a constant level of control pressure via a three position solenoid valve. When the control pressure is applied to the pressure reducing unit, the ball valve is placed into a closed position such that the wheel cylinder is hydraulically disconnected from the master cylinder and the pressure reducing unit performs pressure control operation for increasing and decreasing the braking pressure in the wheel cylinder. It becomes thus possible to apply the braking force to the wheel cylinder even when the brake is not manually operated, for the purpose of performing a traction control operation. In addition, by disconnecting the wheel cylinder from the master cylinder by means of the ball valve, an anti-skid brake control becomes possible by the action of the pressure reducing unit. On the other hand, however, the anti-skid brake control is performed only in the closed circuit, so that it becomes impossible to achieve the pressure boosting function for boosting the master cylinder pressure.