The present invention relates to improvements in a system for controlling the braking system of a vehicle with a hydraulic brake. When the brake fluid pressure in a brake fluid line to the hydraulic brake is controlled by a brake fluid pressure control system, the hydraulic brake is gradually brought into a releasing condition by a self-controlling operation of the hydraulic brake if the brake fluid pressure in the brake fluid line is returned into an actuating condition at the completion of an operation of the brake fluid pressure control system. Therefore, during the self-controlling operation, the hydraulic brake provides only a reduced braking effect as compared with the braking effect provided when the brake fluid pressure is being applied. In view of the above drawback, Japanese patent laid-open publication No. 50-69493 discloses an automatic braking force adjusting device which includes a sensor detecting the operating condition of the hydraulic brake. An automatic pressure adjusting device is controlled by the sensor in such a manner as to gradually increase or decrease the brake fluid pressure in the brake fluid line by controlling an operating condition of the hydraulic brake. This automatic braking force adjusting device is capable of effectively adjusting the braking force to be automatically generated in the hydraulic brake by taking into account the automatic pressure adjusting device, in view of the fact that the automatic braking force adjusting device, is operated in synchronism with the automatic pressure adjusting device. When the brake fluid pressure in the brake fluid line is automatically controlled by the automatic braking force adjusting device during a self-controlling operation of the hydraulic brake, it is desirable that the automatic braking force adjusting device can immediately respond to the increase of the brake fluid pressure in the brake fluid line so that the brake fluid pressure in the brake fluid line can immediately be increased to a level suitable for achieving an effective self-controlling operation. However, in practice, the automatic braking force adjusting device has to be initially operated by an operator and requires a certain period of time before it is completely stabilized. It will therefore be understood that the automatic braking force adjusting device cannot provide a desired braking effect immediately after the operation of the hydraulic brake has been switched over to an automatic control mode by the sensor until the automatic braking force adjusting device has been completely stabilized. To overcome the above mentioned drawback, Japanese patent laid-open publication No. 52-8584 discloses an automatic braking force adjusting device which has a second operating means. The automatic braking force adjusting device includes the sensor detecting the operating condition of the hydraulic brake, and a second sensor for detecting the operation of the second operating means. A solenoid valve is provided in a feed passage extending from the brake fluid pressure control device, and the solenoid valve is controlled by the sensor in such a manner as to gradually increase or decrease the brake fluid pressure in the brake fluid line by controlling the operating condition of the hydraulic brake. In addition, the automatic braking force adjusting device includes a third operating means which is adapted to be operated independently of the first and second operating means, and which is responsive to the operating condition of the hydraulic brake to establish a self-controlling operation of the hydraulic brake. This third operating means is controlled by the sensor in such a manner as to immediately establish an automatic control mode for the automatic braking force adjusting device after the second operating means has been switched to the automatic control mode and to maintain the automatic control mode for the automatic braking force adjusting device at least until the third operating means has been switched to the automatic control mode. It is, however, difficult to provide a suitable operating condition of the third operating means and to stabilize the operation of the third operating means immediately after the operation of the hydraulic brake has been switched to the automatic control mode by the sensor, due to the following reason. When the brake fluid pressure in the brake fluid line is increased immediately after the operation of the hydraulic brake has been switched to the automatic control mode by the sensor, an undesired increase of the brake fluid pressure in the brake fluid line sometimes takes place before the brake fluid pressure is increased to a level sufficient to fully operate the third operating means for a smooth and effective self-controlling operation. It should also be noted that the third operating means may not be operated by a single operating element, but may be operated by a single operating element after the other operating elements have been operated for a certain period of time. Therefore, the time required for the third operating means to be stabilized at an operating condition suitable for an effective self-controlling operation may be shortened if the automatic braking force adjusting device is so arranged that a control operation of the third operating means is commenced before the third operating means has been completely stabilized. It will also be understood that the response of the third operating means cannot be increased simply by increasing the operating speed of the third operating means. An object of the present invention is to provide an improved braking control system of the above mentioned type which can prevent the braking effect generated by the hydraulic brake from being lessened to a significant degree immediately after the operation of the hydraulic brake has been switched over to the automatic control mode. Another object of the present invention is to provide a braking control system of the above mentioned type which can stabilize the braking effect generated by the hydraulic brake without any significant increase of the response time thereof. A further object of the present invention is to provide a braking control system of the above mentioned type which can effectively prevent the response of the third operating means from being adversely affected by a certain operating condition of the second operating means. Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. To achieve the above objects, pursuant to an embodiment of the present invention, a hydraulic pressure control valve is provided in a feed passage extending from a brake fluid pressure control device, and the hydraulic pressure control valve is operated by an operating element to open or close the feed passage. A sensor detects an operating condition of the hydraulic brake and provides a signal indicative of the operating condition of the hydraulic brake, and an automatic braking force adjusting device is operated to increase or decrease the brake fluid pressure in the brake fluid line for generating a braking force in the hydraulic brake. The sensor also detects the operating condition of the operating element, and provides a signal indicative of the operating condition of the operating element. The automatic braking force adjusting device includes a first operating means for increasing or decreasing the brake fluid pressure in the brake fluid line to control the hydraulic brake, and a second operating means for initiating a self-controlling operation of the hydraulic brake to prevent the brake fluid pressure in the brake fluid line from being undesirably decreased to a lower value than a value at which the brake fluid pressure in the brake fluid line has been increased during the automatic controlling operation of the hydraulic brake. The automatic braking force adjusting device further includes a third operating means which is responsive to the operating condition of the hydraulic brake to establish the automatic controlling operation of the hydraulic brake for preventing the brake fluid pressure in the brake fluid line from being undesirably increased. The third operating means is initiated to establish the automatic controlling operation of the hydraulic brake only after the first and second operating means have been initiated, and is arranged to increase the brake fluid