Braking system for braking a motor vehicle

Abstract: A braking system for braking a motor vehicle by a front and a rear brake (20, 30, 202, 204) for a front and a rear wheel of the vehicle, respectively, including a distribution control device (34, 68, 70, 80, 84, 90, 110, 160, 170, 180, 210, 212) for controlling a distribution of a front and a rear wheel braking force produced by the respective front and rear brakes, according to a selected one of a first and a second distribution pattern, each of which represents the front and rear wheel braking forces with respect to each other such that the rear wheel braking force defined by the second distribution pattern is larger than that defined by the first distribution pattern at least when the front and rear wheel braking forces are smaller than respective predetermined values.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates in general to a braking system for braking a motor vehicle, and more particularly to techniques for improving braking performance of a braking system of a motor vehicle by optimizing the distribution of braking forces of front and rear wheels of the vehicle. Discussion of the Related Art The braking forces of the front and rear wheels of a motor vehicle at which these front and rear wheels begin to be locked simultaneously on a road surface upon braking of the vehicle are represented by a curve which is upwardly convex as indicated in the graph of Fig. 1 wherein the braking force of the rear wheel is taken along the vertical axis while the braking force of the front wheel is taken along the horizontal axis. This curve is referred to as "ideal distribution curve", which represents an ideal distribution of the braking forces applied to the front and rear wheels. To improve the braking capacity or performance of the braking system, the specifications of the braking system should desirably be determined such that a curve of the actual distribution of the front and rear wheel braking forces is as close as possible to the ideal distribution curve. The specifications of the braking system include, for example, diameters of the front and rear wheel brake cylinders, effective radii of the front and rear disc brake rotors, and inside diameters of the front and rear wheel brake drums. While the ideal distribution is represented by a curve as described above, the actual distribution of the front and rear wheel braking forces as established by a most basic arrangement of a braking system is represented by a straight line, as indicated in Fig. 1. The most basic arrangement is not provided with a load-sensing proportioning valve (LSP valve). It is also noted that the rear wheel braking force according to the ideal distribution is not constant, but increases as the amount of load acting on the vehicle increases with respect to the minimum load, which is a load acting on the vehicle during a minimum-load run of the vehicle. The "minimum-load run" means a run of the vehicle with only the driver (without any passengers in the case of a passenger car, or without any cargo, luggage or load in the case of an industrial vehicle). The ideal distribution curves representing the ideal distributions during the minimum-load run of the vehicle differs from that during a full-load run of the vehicle, as also indicated in Fig. 1. The "full-load run" means a run of the vehicle with the nominal number of passengers (including the driver) in the case of the passenger car or the nominal maximum load in the case of the industrial vehicle. Further, the braking system is generally designed so as to avoid locking of the rear wheels prior to locking of the front wheels upon braking of the vehicle, for the purpose of preventing the vehicle from losing control of the running direction. The braking system is also designed to prevent the prior locking of the rear wheels with respect to the front wheels during the minimum-load run in which the load acting on the rear wheels is the smallest, causing the rear wheels to having the highest locking tendency. Described in more detail, the braking system is usually adapted so as to minimize a deviation of the actual distribution of the front and rear wheel braking forces (i.e., a deviation of the basic distribution line as determined by the specifications of the most basic braking arrangement indicated above) from the ideal distribution curve, in a direction that causes an increase in the rear wheel braking force. In practice, however, it is difficult to design the braking system so that the actual distribution line of the front and rear wheel braking forces is sufficiently close to the ideal distribution line. As shown in the graph of Fig. 1, the actual distribution line has a larger amount of deviation from the ideal distribution curve during the full-load run of the vehicle than during the minimum-load run. The deviation takes place in the direction that causes a decrease in the rear wheel braking force. That is, the designing of the braking system in an effort to accurately follow the ideal distribution curve has a limitation since the actual basic distribution is generally represented by a straight line and since the ideal distribution curve varies with a change in the vehicle load. While the basic arrangement of the braking system has the drawback as described above, there is also known an improved arrangement in which a proportioning valve is disposed between the hydraulic pressure source and a rear wheel brake cylinder, so that the actual distribution line is made closer to the actual distribution curve. As indicated in Fig. 1, the actual distribution lines of the proportioning valve (load-sensing proportioning or LSP valve) are bent straight lines which are closer to the ideal distribution curve than the basic distribution lines. As disclosed in JP-U-2-130870 the proportioning valve is a kind of pressure reducing valve which is adapted to reduce the hydraulic pressure generated by the hydraulic pressure source at a predetermined ratio and apply the reduced hydraulic pressure as the braking pressure to the rear wheel brake cylinder, after the generated hydraulic pressure has exceeded a predetermined threshold level. Until the generated hydraulic pressure reaches the threshold level (indicated by dots in Fig. 1 at the points of bending of the distribution lines of the proportioning valve), the proportioning valve does not function as the pressure reducing valve, and the hydraulic pressure as generated by the pressure source is applied to the rear wheel brake cylinder. In industrial vehicles such as trucks in which the load acting on the rear wheels varies to a considerable extent with a varying amount of cargo, the braking capacity or performance cannot be sufficient when the load on the rear wheels is relatively large, if the threshold level indicated above is fixed, that is, if the level of the generated hydraulic pressure at which the proportioning valves begins to function as the pressure reducing valve is fixed. In view of this drawback, the braking system for such industrial vehicles is equipped with the proportioning valve of a load-sensing type also known in the art. In the load-sensing type proportioning valve (generally referred to as "LSP valve", or "LSPV"), the threshold level which corresponds to the point of bending of the distribution line of the valve changes with a change in the amount of load on the vehicle. There are two types of load-sensing proportioning valve, that is, a linkage type and a ball type. The linkage type utilizes a fact or phenomenon that the amount of relative displacement between portions of a sprung member and an unsprung member which portions correspond to the rear wheels of the vehicle increases with the load which acts on the rear wheels. Thus, the linkage type is adapted to detect the vehicle load in the form of the relative displacement amount of the sprung and unsprung members. The ball type utilizes a fact or phenomenon that the rear portion of the vehicle body is raised with respect to the front portion as the load on the rear wheels decreases. The ball type uses a ball adapted to roll on an inclined surface whose inclination angle changes with the inclination angle of the vehicle body, so that the ball is seated on a valve seat as a result of rolling. In the ball type, the difficulty of rolling of the ball on the inclined surface indicates the vehicle load. However, the degree of approximation of the distribution line of the load-sensing proportioning valve to the ideal distribution curve is limited. That is, it has been difficult to sufficiently solve the undesirable tendency that the actual distribution line of the load-sensing proportioning valve deviates from the ideal distribution curve, in the direction that causes the rear wheel braking force to be smaller than the ideal value, particularly when the vehicle is in the full-load run, as is apparent from the graph of Fig. 1, wherein hatched area is an area of deviation of the actual rear wheel braking force from the ideal value. Therefore, during the full-load run of the vehicle, the actual braking forces applied to the rear wheels are considerably lower than the ideal value, or cannot be increased to the optimum value. Thus, the use of a load-sensing proportioning valve still suffers from insufficient braking forces of the rear wheels, although it prevents the locking of the rear wheels. Document US-A-4 743 074 discloses a braking system comprising the features of the pre-characterizing clause of claim 1. According to this document, the first or second distribution pattern is selected in dependence on load acting on the vehicle which is detected by a load sensor. Thereby, during the full-load run of the vehicle, the deviation of the actual distribution curve from the ideal distribution curve is reduced. The above problem of difficulty of increasing the rear wheel braking forces to the ideal or optimum value during the full-load vehicle run is present also in an anti-lock braking system adapted to control the braking pressures of the wheels in an anti-lock fashion as well known in the art. The anti-lock control of the braking forces will be described in detail. The braking system is classified into an independent front-rear braking force control type, and a diagonal or X-crossing type. In the independent front-rear braking force control type, a pressure application sub-system including the front right and left wheel brakes is independent of a pressure application sub-system including of the rear right and left wheel brakes. In the X-crossing type, a pressure application sub-system including the front left wheel brake and the rear right wheel brake is independent of a pressure application sub-system including the front right wheel brake and the rear left wheel brake. In an anti-lock braking system of the independent front-rear braking force control type, the front wheel braking pressures and the rear wheel braking pressures are usually regulated independently of each other during an anti-lock control of the braking pressures. In this case, the actual front-rear distribution of the braking forces is not bound by the basic distribution line determined by the specifications of the braking system, but can be changed with a high degree of freedom from the basic distribution line. Accordingly, the actual distribution line can be made sufficiently close to the ideal distribution curve. Therefore, the braking system of the independent front-rear braking force control type does not suffer from the above problem that the rear braking forces cannot be increased sufficiently during the full-load run of the vehicle. In an anti-lock braking system of the X-crossing type, several arrangements are available for effecting the anti-lock control of the braking forces. One example of such arrangements is illustrated in Fig. 2, wherein a normally-open master cylinder cut-off valve 306 is provided in a front brake cylinder passage 304 connecting a master cylinder 300 (hydraulic pressure source) and a front wheel brake cylinder 302, while a rear brake cylinder passage 308 is connected at one end thereto to a portion of the front brake cylinder passage 304 between the cut-off valve 306 and the front wheel brake cylinder 302. The rear brake cylinder passage is connected at the other end to a rear wheel brake cylinder 307. A normally-closed shut-off valve in the form of a pressure reducing valve 312 is provided in a reservoir passage 310 which is connected at one end thereto to the rear brake cylinder passage 308 and at the other end to a reservoir 316, which receives the brake fluid discharged from the wheel brake cylinders 302, 307 through the shut-off valve 312. A pump 318 is connected to the reservoir 316 to return the brake fluid to the master cylinder 300. According to this braking arrangement, the braking pressures in the front and rear wheel brake cylinders 302, 307 are increased by the pressure generated by the master cylinder 300. The applicant of the present application proposed another braking arrangement of the X-crossing type, as shown in Fig. 3. Unlike the braking arrangement of Fig. 2, the present braking arrangement of Fig. 3 is adapted to increase the braking pressures in the the front and rear wheel brake cylinders by operation of the pump 318. That is, the master cylinder cut-off valve 306 is held closed during an anti-lock control of the braking pressures, in principle, and the pump 318 is connected to a portion of the front brake cylinder passage 304 which is downstream of the cut-off valve 306, so that the pressurized fluid from the pump 318 is not returned to the master cylinder 300 but is returned to the wheel brake cylinders 302, 307, whereby the braking pressures in the wheel brake cylinders are increased by operation of the pump 318 during the anti-lock control of the braking system. In either of the two arrangements of the anti-lock braking system of the X-crossing type, the braking pressures in the front and rear wheel brake cylinders 302, 307 cannot be regulated independently of each other, but are regulated such that the braking pressure in the front wheel brake cylinder 302 is equal to that in the rear wheel brake cylinder 307. Therefore, unlike the braking system of the independent front-rear braking force control type, the braking system of the X-crossing type is not capable of establishing the actual distribution line which is shifted from the basic distribution line in the direction that causes an increase in the braking pressure in the rear wheel brake cylinder during the anti-lock control. Thus, like the ordinary braking system incapable of effecting the anti-lock control of the braking forces, the braking system of the X-crossing type suffers from the problem of insufficient braking forces of the rear wheels during the full-load run of the vehicle. The anti-lock braking system of the X-crossing type may take further arrangements as illustrated in figs. 4 and 5. In the arrangement of Fig. 4, two 3-position valves 320 each having a pressure-increase position, a pressure-hold position and a pressure-decrease position are provided for the front and rear wheel brake cylinders 302, 307, respectively. In the arrangement of Fig. 5, a series connection of two shut-off valves 322, 324 is provided for each of the front and rear wheel brake cylinders 302, 307, in place of the 3-position valve 320 used in the arrangement of Fig. 4. Although these arrangements of Figs. 4 and 5 permit the actual distribution of the front and rear wheel braking forces to be controlled without restriction by the basic distribution line, these arrangements have another problem that the construction is inevitably complicated, leading to an increased cost of manufacture. SUMMARY OF THE INVENTION It is a principal object of the present invention to further develop the braking system comprising the features of the pre-characterizing clause of claim 1 such that the braking system permits an improved anti-lock control of the braking forces. It is a first optional object of the present invention to provide an anti-lock braking system of a diagonal or X-crossing type having two pressure application sub-systems each basically designed to effect simultaneous control of the front and rear wheel braking pressures, which braking system permits improved anti-lock control of the braking pressures during a full-load run of the vehicle, in particular, such that the rear wheel braking force is larger than the front wheel braking force, while avoiding complicated construction of a pressure control valve device, It is a second optional object of this invention to provide an anti-lock braking system of a diagonal or X-crossing type having two pressure application sub-systems each having a fluid recirculating pump operated to increase the front and rear wheel braking pressures in an anti-lock pressure control mode, and an intermediate valve for permitting only the front wheel braking pressure to be increased, to assure optimum distribution of the front and rear wheel braking forces. It is a third optional object of the present invention to provide an anti-lock braking system of the X-crossing type in which only the front wheel braking pressure can be increased, and which uses a check valve device simple in construction. It is a fourth optional object of the present invention to provide an anti-lock braking system of the X-crossing type in which only the front wheel braking pressure can be increased, and which has a duty-cycle control mode in which an intermediate valve is alternately closed and opened to control the front and rear wheel braking pressures, with an increased freedom of control. It is a fifth optional object of this invention to provide an anti-lock braking system wherein the duty cycle of the intermediate valve can be changed to thereby further improve the freedom of control of the braking pressures. It is a sixth optional object of the invention to provide an anti-lock braking system wherein the duty cycle of the intermediate valve can be optimally controlled on the basis of the pressure reducing tendencies during the anti-lock pressure control operation. The above principle object is achieved by the braking system according to claim 1. Advantageous further developments of the braking system according to the invention are defined in claims 2 to 12. As is schematically illustrated in Fig. 6, for braking a motor vehicle by operation of a front brake 202 and a rear brake 204 for respective front and rear wheels of the vehicle, the braking system according to the invention includes a distribution control device 210 adapted to control a distribution of a front and a rear wheel braking force which are produced by the front and rear brakes 202, 204, respectively, and which are applied to the front and rear wheels. The distribution control device 210 controls the distribution according to a selected one of a first distribution pattern and a second distribution pattern. Each of the first and second distribution patterns represents the front and rear wheel braking forces with respect to each each other such that the rear wheel braking force defined by the second distribution pattern is larger than that defined by the first distribution pattern at least when the front and rear wheel braking forces are smaller than respective predetermined values. Namely, the first and second distribution patterns are formulated so that the rear wheel braking force according to the second distribution pattern is larger than that according to the first distribution pattern, at least when the vehicle is braked with the relatively small braking forces applied to the front and rear wheels. The present braking system may of course use friction brakes as the front and rear brakes, but may use other types of brakes such as electromagnetic brakes, rheostatic brakes, electro-regenerative brakes and pneumatic brakes. The distribution control device 210 used in the present braking system may take various forms. In the braking system equipped with hydraulically operated friction brakes, for example, the front and rear braking forces may be controlled by regulating the hydraulic braking pressures (pressures of the brake fluid) applied to the friction brakes, or by controlling pressure-receiving areas of pistons which receive the hydraulic braking pressures to force friction members onto rotors rotating with the vehicle wheels. In the latter case, the number of the pistons which are effectively operated for each of the rotors may be suitably selected. In one form of the distribution control device adapted to regulate the hydraulic braking pressures, the front wheel braking pressure and the rear wheel braking pressure are controlled to have the same level according to the first distribution pattern, while the front wheel braking pressure is reduced with respect to the rear wheel braking pressure according to the second distribution pattern. The ratio of the reduction of the front braking pressure with respect to the rear wheel braking pressure may be constant. The amount of the reduction of the front wheel braking pressure may be the same over the entire range in which the front wheel braking pressure varies, and under all running conditions of the vehicle. Alternatively, the reduction amount of the front wheel braking pressure may vary depending upon the vehicle running condition. For instance, the reduction amount of the front wheel braking pressure may be increased with an increase in the friction coefficient of the road surface on which the vehicle runs. The "predetermined value" of the front and rear braking forces may be selected to be a value substantially corresponding a threshold value of a proportioning valve above which the rear braking force is reduced with respect to the front braking force during a minimum-load run of the vehicle. The "predetermined value" may be smaller than the above-indicated value. Alternatively, the "predetermined value" may be selected to be a value substantially corresponding to a threshold value of the proportioning valve during a full-load run of the vehicle. The "predetermined value" may correspond to a value intermediate between the threshold values of the proportioning value during the minimum-load and full-load runs of the vehicle. As indicated above, the principle of the present invention can be satisfied if the distribution control device operates according to the first and second distribution patterns which are formulated such that the rear wheel braking force defined by the second distribution pattern is larger than that defined by the first distribution pattern as long as the front and rear wheel braking forces are smaller than the respective predetermined values. Namely, the principle of the present invention requires the first and second distribution patterns to be formulated such that the rear wheel braking force according to the second distribution pattern is larger than that according to the first distribution pattern, when the vehicle is braked with the relatively small braking forces applied to the front and rear wheels. However, the first and second distribution patterns may be formulated such that the rear wheel braking force defined by the second distribution pattern is larger than that defined by the first distribution pattern, over the entire ranges of the front and rear braking forces. Namely, the first and second distribution patterns may be formulated such that the rear wheel braking force according to the second distribution pattern is larger than that according to the first distribution pattern, irrespective of the levels of the braking forces to be applied to the front and rear wheels. The present braking system is applicable to not only a 4-wheel motor vehicle but also to a 2-wheel motor vehicle, and applicable equally to both a passenger car and an industrial vehicle such as a truck. In the conventional braking system using a load-sensing proportioning valve, the distribution line of the proportioning valve considerably deviates from the ideal distribution curve in the direction that causes the rear wheel braking force to be smaller than the ideal or optimum value, particularly when the vehicle running in the full-load condition is braked with relatively small braking forces applied to the front and rear wheels, as indicated in the graph of Fig. 1. The braking forces applied to the front and rear wheels are relatively small in an initial period of operation of the brake pedal, or during braking on a road surface having a low friction coefficient. Therefore, the conventional braking system is not capable of increasing the rear wheel braking force to a sufficiently high level even where there does not exist a risk of locking of the rear wheel. This drawback arises because the distribution line of the proportioning valve used in the conventional system is the same for both the minimum-load run and the full-load run of the vehicle, as long as the braking forces are relatively small. In this respect, it is noted that the distribution lines of the proportioning lines for the minimum-load run and the full-load run as shown in Fig. 1 coincide with each other at their portions near the zero point of the coordinate system of the graph of Fig. 1. Further, the conventional braking system is adapted so as to prevent locking of the rear wheel prior to the locking of the front wheel. To solve the above drawback, the present braking system is provided with the distribution control device which operates according to the selectively used two distribution patterns which are formulated such that the rear wheel braking force according to the second distribution pattern is larger than that according to the first distribution pattern, at least when the front and rear braking forces are relatively small, that is, smaller than the respective predetermined values. For example, the first and second distribution patterns may be determined such that the first distribution pattern is close or similar to the ideal distribution curve for the minimum-load run of the vehicle while the second distribution pattern is close or similar to the ideal distribution curve for the full-load run of the vehicle. As described above, the minimum-load and full-load runs of the vehicle are the two extreme running conditions of the vehicle in terms of the load acting thereon, which are taken into account in the above case in formulating the first and second distribution patterns of the front and rear wheel braking forces, since the vehicle is usually braked under a load condition between those two extreme load conditions. In summary, the present invention wherein the distribution control device operates according to the two different distribution patterns permits a sufficient increase in the rear wheel braking force without locking of the rear wheel, at least when the braking forces applied to the front and rear wheels are relatively small. Accordingly, the present braking system is capable of reducing the required braking distance of the vehicle while avoiding the prior locking of the rear wheel. In an anti-lock braking system, the braking pressures are regulated in an anti-lock manner so as to prevent locking of the wheels, even if the non-anti-lock control distribution pattern of the front and rear wheel braking forces which will be normally established without the anti-lock control deviates from the ideal distribution pattern which corresponds to the actual vehicle load. Therefore, the actual distribution pattern can accurately coincide with the ideal distribution pattern corresponding to the actual vehicle load, irrespective of whether the vehicle is in the minimum-load run or full-load run, even if the rear wheel braking force according to the non-anti-lock control distribution pattern is relatively large. Based on the above finding, the braking system according to the principle of the present invention may be adapted to effect an anti-lock control of the braking forces according to a first preferred form of this invention, which is suitable for achieving the first optional object indicated above. In this first preferred form of the invention, the braking system is an anti-lock braking system of a diagonal or X-crossing type for a four-wheel motor vehicle, as shown in a hydraulic circuit diagram of Fig. 7. The present braking system has two pressure application sub-systems which are connected to respective two mutually independent pressurizing chambers of a hydraulic pressure source. The hydraulic pressure source may be principally constituted by a master cylinder 224 as is shown in Fig. 7 which mechanically generates a hydraulic pressure commensurate with an operating force acting on a brake operating member. Alternatively, the hydraulic pressure source 200 may be an electrically operated pressure source which electrically generates a hydraulic pressure commensurate with the operating force or operating amount of the brake operating member. The electrically operated pressure source may be principally constituted by a pump and a solenoid-operated pressure control valve which regulates the delivery pressure of the pump. Alternatively, the electrically operated pressure source may use an electric motor whose rotary motion is converted by a ball screw into a linear motion of a piston for generating a hydraulic pressure. In this case, a controller is provided to control the motor to regulate the hydraulic pressure generated by the linear motion of the piston. Each of the two pressure application sub-systems includes (a) a front brake cylinder passage 226 connecting a corresponding one of the two pressurizing chambers of the master cylinder and a front wheel brake cylinder 206 of the front brake, (b) a rear brake cylinder passage 228 connecting the front brake cylinder passage 226 and a rear wheel brake cylinder 208 of the rear brake, (c) a master cylinder cut valve 230 in the form of a normally-open shut-off valve disposed in a portion of the front brake cylinder passage 226 between the master cylinder 224 and a point of connection of the front and rear brake cylinder passages 226, 228, (d) a reservoir passage 234 connected at one of opposite ends thereof to the rear brake cylinder passage 228, (e) a reservoir 232 connected to the other end of the reservoir passage, (f) a pressure reducing valve 236 in the form of a normally-closed shut-off valve disposed in the reservoir passage 234, (g) a pump passage 238 connected at one of opposite ends thereof to the reservoir 232 and at the other end to at least one of the front and rear brake cylinder passages 226, 228, (h) a pump 240 disposed in the pump passage 238 for delivering a working fluid from the reservoir to a portion of the each pressure application sub-system, and (i) a controller 241 operable in an anti-lock pressure control mode for controlling the master cylinder cut valve 230, the pressure reducing valve 236 and the pump to effect an anti-lock pressure control operation for controlling pressures of the fluid in the front and rear wheel brake cylinders 206,208 in an anti-lock manner, and wherein the distribution control device comprises the controller 241, and a pressure reduction control device 242 disposed in a portion of each pressure application sub-system which is other than the portion between the master cylinder and the connection of the front and rear brake cylinder passages. The pressure reduction control device is adapted to apply to the front wheel brake cylinder 206 the pressure as generated by a first hydraulic pressure source in the form of the master cylinder 224, to thereby establish the first distribution pattern, when the controller 241 is not placed in the anti-lock pressure control mode. The controller 241 and the pressure reduction control device 242 cooperate to establish the second distribution pattern such that the pressure generated by a second hydraulic pressure source which consists of at least one of the master cylinder 224 and the pump 240 is reduced by the pressure reduction control device 242 and is then applied to the front wheel brake cylinder 208, when the controller 241 is placed in the anti-lock pressure control mode. The anti-lock braking system constructed as described above may be desirably provided with a proportioning valve adapted to reduce the pressure in the rear wheel brake cylinder 208 with respect to the pressure in the front wheel brake cylinder 205 after the pressure generated by the hydraulic source (master cylinder or pump) becomes higher than the front wheel brake cylinder 205 by more than a predetermined difference, which may be either fixed or variable. In this instance, the proportioning valve may be disposed in a portion of the rear brake cylinder passage 228 between the rear wheel brake cylinder 208 and the point of connection of the rear brake cylinder passage 228 and the pump passage 238. In this case, means for disabling the proportioning valve during an operation of the pump 240 to increase the rear wheel brake cylinder pressure. This disabling means prevents the proportioning valve from operating to reduce the rear wheel brake cylinder pressure while it is required to increase the rear wheel brake cylinder pressure by operation of the pump 240. In the anti-lock braking system illustrated in Fig. 7, the pump 240 functions in principle as the hydraulic pressure source during the anti-lock pressure control of the front and rear wheel brake cylinders 206, 208. However, the master cylinder 224 may be used as the hydraulic pressure source when the pump 240 is required to be operated to increase the front and rear wheel brake cylinder pressures while no working or brake fluid is stored in the reservoir 232. In this exceptional case, the master cylinder cut valve 230 is opened, to permit the fluid pressurized by the master cylinder 224 to be supplied to the front and rear wheel brake cylinders 206, 208 to thereby increase the pressures in these brake cylinders. In the present first preferred form of the invention, therefore, at least one of the master cylinder 224 and the pump 240 is used as the hydraulic pressure source during the anti-lock pressure control of the front and rear wheel brake cylinders 206, 208. Although the braking system of Fig. 7 is adapted such that the delivery or output end of the pump passage 238 is connected to the rear brake cylinder passage 228 which is downstream of the master cylinder cut valve 230, the output end of the pump passage 230 may be connected to a portion of the front brake cylinder passage 226 which is upstream or downstream of the master cylinder cut valve 230, for example. While the pressure reduction control device 242 is disposed in the rear brake cylinder passage 228 in the braking system of Fig. 7, the pressure reduction control device 242 may be disposed in the front brake cylinder passage 226. In the braking system of Fig. 7, a by-pass passage is provided to by-pass the master cylinder cut valve 230, and a check valve 250 is disposed in this by-pass passage, so as to inhibit a flow of the fluid from the master cylinder 224 toward the front and rear wheel brake cylinders 206, 208. The check valve 250 permits a flow of the fluid in the opposite direction, with the valve opening pressure difference being substantially zero. This check valve 250 functions not only as a valve for permitting a rapid return of the fluid to the master cylinder 224 upon releasing of the brake operating member, but also as a release valve for preventing an excessive rise of the pressure in the front wheel brake cylinder 206 above the pressure level of the master cylinder 200 during an operation of the pump 240 to increase the front wheel brake cylinder pressure. Reference numeral 252 also denotes a check valve for the rear wheel brake cylinder 208. The check valve 252 has the same function as the check valve 250 for the front wheel brake cylinder 206. In the anti-lock braking system of the diagonal or X-crossing type having the two pressure application sub-systems according to the first preferred form of the invention, each pressure application sub-system is basically designed to effect simultaneous control of the front and rear wheel braking pressures. During the normal operation of the system without the anti-lock pressure control, the pressure as generated by the pressure source is applied to the front wheel brake cylinder, whereby the front and rear wheel braking forces are controlled according to the first distribution pattern. During the anti-lock pressure control, the pressure generated by the pressure source is reduced by the pressure reduction control device before it is applied to the front wheel brake cylinder. Thus, the controller and the pressure reduction control device cooperate to establish the second distribution pattern so that the pressure in the front wheel brake cylinder is smaller than that in the rear wheel brake cylinder. The anti-lock braking system according to the above first preferred form of the invention has an advantage as described below by reference to the graphs of Fig. 9. While the present anti-lock braking system is in the normal pressure control mode, the distribution of the front and rear wheel braking forces is controlled according to the first distribution pattern, regardless of whether the vehicle is in the minimum-load run or in the full-load run. As a result, the actual distribution pattern or curve during the full-load run deviates from the ideal distribution curve such that the rear wheel braking force BR is smaller than the optimum or ideal value, as indicated at right in Fig. 9(a). While the braking system is in the anti-lock pressure control mode, the actual distribution curve established as a result of the anti-lock pressure control is sufficiently close and similar to the ideal distribution curve during the minimum-load run, even if the non-anti-lock control distribution curve deviates from the ideal distribution curve such that the rear wheel braking force BR is larger than the ideal value. During the full-load run, on the other hand, the non-anti-lock control distribution curve is sufficiently close and similar to the ideal distribution curve (for the full-load run), namely, does not deviate from the ideal distribution curve such that the rear wheel braking force BR is larger than the ideal value. Accordingly, the actual distribution curve is sufficiently close and similar to the non-anti-lock control distribution curve. Thus, the actual distribution curve substantially coincides with the ideal distribution curve as indicated in Fig. 9(b), irrespective of the minimum-load run or full-load run of the vehicle, owing to the anti-lock pressure control operation and owing to the non-anti-lock control distribution. In the present first preferred form of the invention, the first distribution pattern is established in the normal pressure control mode irrespective of the minimum-load or full-load run of the vehicle, and during the minimum-load run in the anti-lock pressure control mode, while the second distribution pattern is established during the full-load run in the anti-lock pressure control mode. The present first preferred form may be suitably the first distribution pattern is established during the minimum-load run in both the normal pressure control mode and the anti-lock pressure control mode, while the second distribution pattern is established during the full-load run in both of the normal and anti-lock pressure control modes. It will be understood from the above description that the anti-lock braking system according to the first preferred form of the invention assures the actual distribution pattern of the front and rear wheel braking forces which is as close as possible to the ideal distribution curve or pattern in the anti-lock pressure control mode, whereby the required braking distance of the vehicle in the anti-lock pressure control mode can be significantly reduced. The second optional object indicated above may be achieved according to a second preferred form of the present invention, wherein the braking system is an anti-lock braking system of a diagonal or X-crossing type for a four-wheel motor vehicle, as illustrated in Fig. 8. Each pressure application sub-system has two pressure application sub-systems which are connected to respective two mutually independent pressurizing chambers of a master cylinder 224, each of the two pressure application sub-systems including (a) a front brake cylinder passage 226 connecting a corresponding one of the two pressurizing chambers of the master cylinder 224 and a front wheel brake cylinder 206 of the front brake, (b) a rear brake cylinder passage 228 connecting the front brake cylinder passage 226 and a rear wheel brake cylinder 208 of the rear brake, (c) a master cylinder cut valve 230 in the form of a normally-open shut-off valve disposed in a portion of the front brake cylinder passage 226 between the master cylinder 224 and a point of connection of the front and rear brake cylinder passages 226, 228, the master cylinder cut valve 230 being closed when the braking system is in an anti-lock pressure control mode, and opened when the braking system is not in the anti-lock pressure control mode, (d) an intermediate valve 254 in the form of a normally-open shut-off valve disposed in the rear brake cylinder passage 228, (e) a reservoir passage 234 connected at one of opposite ends thereof to a portion of the rear brake cylinder passage 228 between the intermediate valve 254 and the rear wheel brake cylinder 228, (f) a reservoir 232 connected to the other end of the reservoir passage 234, (g) a pressure reducing valve 236 in the form of a normally-closed shut-off valve disposed in the reservoir passage 234, (h) a pump passage 238 connected at one of opposite ends thereof to the reservoir 232 and at the other end to a portion of the rear brake cylinder passage 228 between the intermediate valve 254 and a point of connection of the front and rear brake cylinder passages 226, 228, (i) a pump 240 disposed in the pump passage 238 for delivering a working fluid from the reservoir 232 to a portion of the each pressure application sub-system, and (j) a controller 241 operable in the anti-lock pressure control mode for controlling the master cylinder cut valve 230, the intermediate valve 254, the pressure reducing valve 236 and the pump 240 to effect an anti-lock pressure control operation for controlling pressures of the fluid in the front and rear wheel brake cylinders 206, 208 in an anti-lock manner, and wherein the distribution control device comprises the controller 241, and a check valve device 256 disposed in a portion of the rear brake cylinder passage 228 between the point of connection of the front and rear brake cylinder passages 226, 228 and a point of connection of the rear brake cylinder passage 228 and the pump passage 238, the check valve device comprising a first check valve 258) and a second check valve 260, the first check valve permitting a flow of the fluid therethrough in a first direction from the pump 240 toward the front wheel brake cylinder 206 after the pressure generated by the pump becomes higher than the pressure in the front wheel brake cylinder 206 by more than a predetermined difference, and inhibiting a flow of the fluid therethrough in a second direction opposite to the first direction, the second check valve 260 permitting a flow of the fluid therethrough in the second direction and inhibiting a flow of the fluid therethrough in the first direction. The check valve device used in the second preferred form of the invention can be considered one example of the pressure reduction control device. In the above form of the anti-lock braking system, the master cylinder cut valve 230, pressure reducing valve 236 and intermediate valve 254 are provided to control the pressures in the front and rear wheel brake cylinders. As shown in fig. 8, the intermediate valve 254 is disposed between the front and rear wheel brake cylinders 206, 208, to partially introduce independent pressure control of the front and rear wheel brake cylinder pressures into the basic arrangement for simultaneous control of these brake cylinder pressures. In the absence of the intermediate valve 254, the front and rear wheel brake cylinder pressures would always be controlled in the same pressure control manner or mode, and these pressures would have the same relationship. In the presence of the intermediate valve 254, the front and rear wheel brake cylinder pressures can be regulated independently of each other, depending upon the locking tendencies of the corresponding front and rear wheels. For example, the front wheel brake cylinder pressure can be increased while the rear wheel brake cylinder pressure can be reduced. The present anti-lock braking system is adapted such that the output end of the pump passage 238 is connected to a portion of the rear brake cylinder passage 228 which is upstream of the intermediate valve 254. In this arrangement, the front wheel brake cylinder pressure is increased while the rear wheel brake cylinder pressure is reduced or held constant, if the pump 24o is operated to deliver the pressurized fluid while the intermediate vale 254 is held closed. Thus, the front wheel brake cylinder pressure can be increased independently of the rear wheel brake cylinder pressure. Therefore, this arrangement is effective to reduce the required braking distance where the vehicle is braked when the front and rear wheels lie on respective areas of the road surface which have relatively high and low friction coefficient values, respectively. Namely, the present arrangement makes it possible to maximize the front wheel brake cylinder pressure to maximize the utilization of the relatively high friction coefficient of the road surface area on which the front wheel lies. In the above anti-lock braking system of Fig. 8, the check valve device 256 of the distribution control device is disposed in the portion of the rear brake cylinder passage 228 between the points of connection to the front brake cylinder passage 226 and the pump passage 238. Further, the check valve device 256 comprises the first and second check valves 258, 260 which are arranged in parallel to each other, such that the first check 258 permits the fluid flow in the first direction from the pump 240 toward the front wheel brake cylinder 206 only after the delivery pressure of the pump 240 becomes higher than the pressure in the front wheel brake cylinder 206 by more than the predetermined difference, while the second check valve 260 permits the fluid flow in the second direction opposite to the first direction, with the valve opening pressure difference being substantially zero. While the braking system of Fig. 8 or its controller 241 is not placed in the anti-lock pressure control mode, that is, placed in the normal pressure control mode, the fluid as pressurized by the master cylinder 224 is supplied to the front wheel brake cylinder 206 through the open master cylinder cut valve 230, and to the rear wheel brake cylinder 208 through the second check valve 260 and the normally-open intermediate valve 254, without pressure reduction by the check valve device 256. Thus, the pressure applied to the rear wheel brake cylinder 208 is not reduced with respect to the master cylinder pressure, whereby the distribution of the front and rear wheel braking forces is controlled according to the first distribution pattern which causes the front and rear wheel brake cylinder pressures to be equal to each other. While the controller 241 is placed in the anti-lock pressure control mode, the master cylinder cut valve 230 is closed and the pump 240 is operated, so that the fluid delivered from the pump 240 is applied to the front wheel brake cylinder 206 through the first check valve 258. Since the fluid is permitted to flow through the first check valve 258 in the first direction only after the delivery pressure of the pump 240 has been raised to the predetermined valve opening threshold level, the pressure to be applied to the front wheel brake cylinder 206 is reduced to the delivery pressure of the pump 240 by an amount corresponding to the opening pressure difference of the first check valve 258. However, the fluid as delivered from the pump 240 is applied to the rear wheel brake cylinder 208, without pressure reduction by the first check valve 258. Accordingly, the pressure in the front wheel brake cylinder 206 is controlled to be lower than that in the rear wheel brake cylinder 208 by the amount corresponding to the opening pressure difference of the first check valve 258, while the front and rear wheel brake cylinder pressures are increased in the anti-lock pressure control mode with the pump 240 being operated. Therefore, the distribution of the front and rear wheel braking forces in the anti-lock pressure control mode is controlled according to the second distribution pattern, which is substantially the same as the non-anti-lock control distribution pattern which is established in the normal pressure control mode, that is, the pattern which causes the rear wheel braking force to be relatively high. The distribution control device constructed according to the second preferred form of the invention may also be used in the first preferred form of the invention described above. In the anti-lock braking system of the X-crossing type according to the present second preferred form of the invention, the provision of the intermediate valve permits the front and rear wheel brake cylinder pressures to be controlled independently of each other in each pressure application sub-system, which is basically designed for simultaneous control of the front and rear wheel brake cylinder pressures. Thus, the intermediate valve provides for improved freedom of anti-lock pressure control of the front and rear wheel brake cylinder pressures, depending upon varying running condition of the vehicle. The third optional object indicated above can be achieved according to one advantageous arrangement of the above second preferred form of the present invention which includes the check valve device. In this arrangement, at least a part of the portion of the rear brake cylinder passage 228 between the point of connection of the front and rear brake cylinder passages 226, 228 and the point of connection of the rear brake cylinder passage 228 and the pump passage 238 consists of a first and a second passage which are concentric with and mutually independent of each other and which have a circular and an annular cross sectional shape, respectively. The first check valve 256 is disposed in one of these first and second passages while the second check valve 260 is disposed in the other of the first and second passages. The first check valve 258 may include a valving member in the form of a ball which is seated on a valve seat under a biasing action of suitable biasing means. The second check valve 260 may be a valve which includes an annular one-way sealing member made of an elastic material. In the above advantageous arrangement, the first and second check valves 258, 260 are disposed in the one and the other of the first and second passages which have the circular and annular cross sectional shapes and which are disposed concentrically with each other such that one of the circular and annular passages is located within the other passage. Namely, the circular and annular passages are not arranged in parallel in the diametric direction thereof. This arrangement is effective to minimize the maximum dimensional of the check valve device 256 in the diametric direction of those first circular and second annular passages, whereby the overall size of the check valve device 256 can be effectively reduced. Accordingly, the braking system incorporating the check valve device can be manufactured without a considerable increase in the required installation space. The fourth optional object indicated above may be achieved according to another advantageous arrangement of the above second preferred form of this invention, wherein the controller 241 has a plurality of pressure control modes which are selectively established to control the master cylinder cut valve 230, the intermediate valve 254 and the pressure reducing valve 236 in the anti-lock manner. The pressure control modes include (1) a mode in which the intermediate valve and the pressure reducing valve are both open while the master cylinder cut valve is closed, to reduce the pressures in both of the front and rear wheel brake cylinders 206, 208, (2) a mode in which the master cylinder cut valve and the intermediate valve are both closed while the pressure reducing valve is open, to increase the pressure in the front wheel brake cylinder and reduce the pressure in the rear wheel brake cylinder, by operation of the pump 240, and (3) a duty-cycle pressure control mode in which the master cylinder cut valve and the pressure reducing valves are both closed while the intermediate valve is alternately closed and opened to increase the pressures in the front and rear wheel brake cylinders by operation of the pump 240. In the above advantageous arrangement, the controller 241 has the duty-cycle control mode in which the intermediate valve 254 is alternately closed and opened to increase the pressures in the front and rear wheel brake cylinders by operation of the pump 240. In the anti-lock pressure control mode, the fluid delivered from the pump 240 is supplied to only the front wheel brake cylinder 206 while the intermediate valve 254 is in the closed state, and is supplied to not only the front wheel brake cylinder 206 but also the rear wheel brake cylinder 208 while the intermediate valve 254 is in the open state. Accordingly, the repetition of the alternate closing and opening of the intermediate valve 254 makes it possible to increase the pressure in each of the front and rear wheel brake cylinders at a rate which is intermediate between a rate when the intermediate valve is kept open and a rate when the intermediate valve is kept closed. The duty-cycle control mode improves the freedom of control of the front and rear wheel braking pressures. The fifth optional object indicated above may be achieved if the controller 241 is adapted to comprise means for changing a duty cycle of the intermediate valve 254 in the duty-cycle pressure control mode. Although the duty cycle may be fixed, it is desirable to change the duty cycle depending upon the specific running condition of the vehicle, so as to control the braking pressures with a higher degree of freedom and thereby enhance the braking performance of the braking system. This arrangement permits the front and rear wheel braking pressures to be increased at optimum rates with the duty cycle of the intermediate valve being suitably controlled in the anti-lock pressure control mode, depending upon the running condition of the vehicle The duty cycle of the intermediate valve 254 may be changed by changing a ratio of the time period during which the intermediate valve is open to the time period during which the intermediate valve is closed. The rates of increase of the wheel braking pressures may be changed by some mechanical means, for instance, flow restrictors such as orifices which are disposed in the front and rear brake cylinder passages and whose cross sectional areas of fluid flow are variable. In the desirable arrangement indicated above, such mechanical means is replaced by the means for changing the duty cycle of the intermediate valve as described above, which means is constituted by a control program or electronic circuitry provided in the controller. Thus, the present arrangement permits the rates of increase of the brake cylinder pressures while avoiding an increase in the cost of the braking system. Generally, individual vehicles have different optimum increase rates of the wheel brake cylinder pressures that assure adequate anti-lock control of the wheel braking forces in relation to the specific characteristics of the braking system such as the ratio of the diameters of the front and rear wheel brake cylinders, and in relation to the specific braking conditions of the vehicle such as the actual braking effect and load distribution of the vehicle on the front and rear wheels. In the above arrangement, the rates of pressure increase of the front and rear wheel brake cylinders can be easily controlled without a costly mechanism, such that the duty cycle of the intermediate valve is controlled by the controller, so as to meet the specific characteristics of the braking system of the particular vehicle. The sixth optional object indicated above may be achieved if the above-indicated means for changing the duty cycle of the intermediate valve is adapted to change the duty cycle on the basis of at least one of pressure reducing tendency of the front wheel brake cylinder and pressure reducing tendency of the rear wheel brake cylinder, which tendencies have been exhibited in the anti-lock pressure control operation under the control of said controller. For example, the pressure reducing tendencies or hystereses are expressed by the numbers or frequencies of pressure reductions which have been effected with the front and rear wheel brake cylinders, the pressure reducing time periods of these cylinders, or the rates at which the pressure have been reduced. Such previous and present pressure reducing tendencies directly reflect the locking tendencies of the front and rear wheels. Accordingly, the locking tendencies of the wheels can be detected by monitoring the pressure reducing tendencies in the anti-lock pressure control mode. If the pressure in the rear wheel brake cylinder has been reduced more frequently than that in the front wheel brake cylinder, it is possible to determine that the rear wheel has a higher locking tendency that the front wheel. In this case, it is desirable to reduce the rear brake cylinder pressure and increase the front wheel brake cylinder pressure. In other words, it is desirable to determine the duty cycle of the intermediate valve, so as to establish a distribution of the pressures of the front and rear wheel braking cylinders which causes a higher rate of increase in the pressure of the front wheel brake cylinder and a lower rate of increase in the rear wheel brake cylinder when the rear wheel brake cylinder has exhibited a higher tendency of pressure reduction that the front wheel brake cylinder. In the above arrangement in which the duty cycle is changed on the basis of the pressure reducing tendencies of the front and rear wheel brake cylinders, the braking capacities of the wheels can be maximized with the locking tendency of each wheel taken into consideration. In this respect, it is noted that the locking tendencies of the wheels are resultant from various factors such as the friction coefficients of the road surface areas on which the wheels lie, and from the wheel braking torques and loads acting on the wheels. Therefore, the rates of increase of the brake cylinder pressures which are determined by the thus controlled duty cycle of the intermediate valve can be adequately controlled with high precision so as to reflect the actual braking or locking tendencies of the wheels. The pressure reducing tendencies of the front and rear wheel brake cylinders can be detected by monitoring the signals generated to energize or de-energize the solenoids of the master cylinder cut-off valve, intermediate valve and pressure reducing valve. Thus, the above arrangement does not require an exclusive sensor for detecting the pressure reducing tendencies of the wheel brake cylinders, and is therefore available at a relatively low cost while it is capable of adequately controlling the duty cycle of the intermediate valve, namely, the pressure increase rates of the wheel brake cylinders. The means for changing the duty cycle of the intermediate valve may be adapted to change the duty cycle on the basis of an amount of shift of a load on the vehicle in a running direction of the vehicle. The load shift may be detected on the basis of the deceleration of the vehicle in the running direction. Described more specifically, upon braking of the vehicle, the load acting on the front wheels increases while the load acting on the rear wheels decreases, due to a shift of the vehicle load in the vehicle running direction. This means that the front wheel brake cylinder pressure should be increased to increase the braking force on the front wheel while the rear wheel brake cylinder pressure should be reduced to prevent the rear wheel from locking. To this end, it is preferable to determine the duty cycle of the intermediate valve, so as to establish a distribution of the pressures of the front and rear wheel brake cylinders which causes a higher rate of increase in the pressure of the front wheel brake cylinder and a lower rate of increase in the rear wheel brake cylinder when the amount of shift of the load to a front wheel for which the front wheel brake cylinder is provided is relatively large than when the amount of shift of the load to the front wheel is relative small. Alternatively, the means for changing the duty cycle of the intermediate valve may be adapted to change the duty cycle on the basis of an amount of shift of a load on the vehicle in the lateral direction of the vehicle, which may be detected on the basis of the lateral acceleration of the vehicle. In the pressure application sub-system in which the front and rear wheels to be braked by the front and rear wheel brake cylinder are respectively located on the outer and inner sides of the vehicle turning line along which the vehicle is turning, the load acting on the front wheel increases while the load acting on the rear wheel decreases, due to a shift of the vehicle load in the lateral direction perpendicular to the running direction. In this case, too, it is desired to increase the front wheel brake cylinder pressure to increase the braking force of the front wheel and decrease the rear wheel brake cylinder pressure to prevent the locking of the rear wheel. In this respect, it is preferable to determine the duty cycle of the intermediate valve, so as to establish a distribution of the pressures of the front and rear wheel brake cylinders which causes a higher ate of increase in the pressure in the front wheel brake cylinder and a lower rate of increase in the rear wheel brake cylinder when the amount of shift of the load is relatively large than when the amount of shift of the load is relatively small, in the pressure application sub-system in which the front and rear wheels are respectively located on the outer and inner sides of the vehicle turning line. BRIEF DESCRIPTION OF THE DRAWINGS The above and optional objects, features and advantages of the present invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings, in which: Fig. 1 is a graph explaining a relationship between braking forces of front and rear wheels in a known braking system of a motor vehicle; Fig. 2 is a schematic view of one example of a known anti-lock braking system of diagonal or X-crossing type; Fig. 3 is a schematic view of another example of the known anti-lock braking system of the same type as that of Fig. 2; Fig. 4 is a schematic view of one example of a known anti-lock braking system of independent front-rear braking force control type; Fig. 5 is a schematic view of another example of the known anti-lock braking system of the same type as that of Fig. 4; Fig. 6 is a block diagram illustrating a principle of the present invention; Fig. 7 is a hydraulic circuit diagram schematically showing one preferred form of the present invention; Fig. 8 is a hydraulic circuit diagram schematically showing a further preferred form of the invention; Fig. 9 is a graph explaining an advantage of the preferred form of the invention of Fig. 7; Fig. 10 is a schematic view illustrating an anti-lock braking system according to one embodiment of this invention; Fig. 11 is a front elevational view in cross section of a proportioning valve provided in the braking system of Fig. 10; Fig. 12 is a hydraulic circuit diagram schematically showing flows of a brake fluid to and from a master cylinder, a pump and front and rear wheel brake cylinders in the braking system of Fig. 16; Fig. 13 is a graph explaining changes in the front and rear wheel brake cylinder pressures as controlled differently in an anti-lock fashion in fourth and fifth modes of operation of the braking system of Fig. 10; Fig. 14 is a graph explaining a relationship between the braking forces of the front and rear wheels as controlled in the braking system of Fig. 10; Fig. 15 is a front elevational view in cross section showing details of construction of a check valve device and a second shut-off valve which are provided in the embodiment of Fig. 10; Fig. 16 is a flow chart illustrating a routine executed by a computer of a controller used in the braking system of Fig. 10, for controlling the second shut-off valve; Fig. 17 is a flow chart illustrating a sub-routine executed in step S40 of the routine of Fig. 16; Fig. 18 is a flow chart illustrating a routine executed by the computer for determining an OFF time T 1 of the solenoid of the second shut-off valve; Fig. 19 is a graph explaining a relationship between intermittent fluid delivery of a pump used in the braking system of Fig. 10 and energization and de-energization of the solenoid of the second shut-off valve; Fig. 20 is a schematic view showing an anti-lock braking system according to a further embodiment of the present invention; Fig. 21 is a front elevational view in cross section of a proportioning valve used in the braking system of Fig. 20; Fig. 22 is a graph explaining a relationship between the braking forces of the front and rear wheels as controlled in the braking system of Fig. 20; and Fig. 23 is a schematic view showing a yet further embodiment of the anti-lock braking system of this invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to Figs. 10 - 15, there will be described a first embodiment of the present invention in the form of an anti-lock braking system of the diagonal or X-crossing type, which has various features. This braking system is capable of effecting an anti-lock control of the braking pressures or forces of the front and rear wheels, and is adapted to use not only the master cylinder but also a fluid recirculating pump as the hydraulic pressure source. In Fig. 10, reference numeral 10 denotes a master cylinder which functions as a hydraulic pressure source. The master cylinder 10 is of a tandem type in which two mutually independent fluid pressurizing chambers are disposed in series. The master cylinder 10 is linked with a brake operating member in the form of a brake pedal 14 through a booster 12. Upon operation or depression of the brake pedal 14 by the driver or operator of the motor vehicle, equal fluid pressures are mechanically generated in the two pressurizing chambers of the master cylinder 10. One of the pressurizing chambers of the master cylinder 10 is connected to brake cylinders of hydraulically operated brakes for a front left wheel and a rear right wheel of the vehicle, while the other pressurizing chambers is connected to brake cylinders of hydraulically operated brakes for a front right wheel and a rear left wheel of the vehicle. These brake cylinders are hereinafter referred to as "wheel brake cylinders". Thus, the braking system has two mutually independent pressure application sub-systems one of which has the front left wheel brake cylinder and the rear right wheel brake cylinder, and the other of which has the front right wheel brake cylinder and the rear left wheel brake cylinder. Since the two pressure application sub-systems are identical in construction with each other, only one of these sub-systems is illustrated in Fig. 10 and will be described below. In each pressure application sub-system, the corresponding pressurizing chamber of the master cylinder 10 is connected to the front wheel brake cylinder 20 through a front brake cylinder passage 22. A rear brake cylinder passage 24 is connected at one end thereof to the front wheel brake cylinder passage 22 and at the other end to the rear wheel brake cylinder 30. As shown in Fig. 10, a normally-open first solenoid-operated shut-off valve 100 is disposed in a portion of the front brake cylinder passage 22 between the master cylinder 10 and the point of connection of the front and rear brake cylinder passages 22, 24. Further, a by-pass return passage 102 is provided in parallel with the first shut-off valve 100, so as to by-pass the shut-off valve 100. The by-pass return passage 102 is provided with a check valve 104, which inhibits a flow of the brake fluid in a direction from the master cylinder 10 toward the front wheel brake cylinder 20, and permits a flow of the brake fluid in the reverse direction with the valve opening pressure difference being substantially zero. In the rear brake cylinder passage 24, there is provided a proportioning valve 110 (hereinafter referred to as "P valve 110"). The P valve 110 is a pressure reducing valve which operates so that the fluid pressure as generated by the master cylinder 10 (hereinafter referred to as "master cylinder pressure") is applied to the rear wheel brake cylinder 30 without reduction of the master cylinder pressure, until the master cylinder pressure reaches a predetermined threshold level, and so that the master cylinder pressure higher than the threshold level is reduced at a predetermined ratio, so that the reduced pressure is applied as the braking pressure to the rear wheel brake cylinder 30. The P valve 110 is not of a load-sensing type having a variable threshold pressure for initiation of the pressure reducing function, but of a fixed threshold type wherein the pressure reducing function is initiated at a predetermined threshold level of the master cylinder pressure. The P valve 110 will be described in detail with reference to Fig. 11. The P valve 110 has a housing 112, which has a stepped cylinder bore 118 with a large-diameter portion 114 and a small-diameter portion 116. A stepped valve piston 124 having a large-diameter portion 120 and a small-diameter portion 122 is slidably received in the stepped cylinder bore 118. The valve piston 124 is biased by biasing means in the form of a spring 126 so that the piston 124 is normally held in a non-operated position in which the end face of the large-diameter portion 120 abuts on the bottom wall of the small-diameter portion 116 of the housing 112. Between the cylinder bore 118 and the valve piston 124, there is disposed a sealing member in the form of a cup seal 128. This cup seal 128 divides the space within the cylinder bore 118 into two sections. One of these two sections which is on the side of the large-diameter portion 114 serves as an input chamber 130, while the other section on the side of the small-diameter portion 116 serves as an output chamber 132. The input chamber 130 is connected to the master cylinder 10, while the output chamber 132 is connected to the rear wheel brake cylinder 30. The cup seal 128 consists of a one-way sealing portion 134 and a two-way sealing portion 136. The one-way sealing portion 134 inhibits a flow of the brake fluid in the direction from the input chamber 130 toward the output chamber 132 while the one-way sealing portion 134 is in fluid-tight contact with the circumferential surface of the large-diameter portion 114 of the cylinder bore 118. The one-way sealing portion 134 permits a flow of the fluid in the direction from the output chamber 132 toward the input chamber 130 while the sealing portion 134 is spaced apart from the surface of the large-diameter portion 114. When the valve piston 124 is moved from the non-operated position of Fig. 11 to an operated position (in the right direction as seen in the figure), the shoulder surface between the large-diameter and small-diameter portions 120, 122 of the piston 124 is brought into abutting contact with the two-way sealing portion 136, thereby inhibiting flows of the fluid in the opposite directions between the input and output chambers 130, 132. When the valve piston 124 is placed in the non-operated position of Fig. 11, the two-way sealing portion 136 is unseated off the shoulder surface of the piston 124, whereby the fluid is permitted to flow between the input and output chambers 130, 132. The cup seal 128 has an annular protrusion formed on each of the opposite surfaces which define the input and output chambers 130, 132. The annular protrusions have a semi-circular cross sectional shape as seen in Fig. 11. The annular protrusion on the side of the input chamber 130 prevents the cup seal 128 from contacting the valve piston 124 at the entire area of the surface on the side of the input chamber 130, while the annular protrusion on the side of the output chamber 132 prevents the cup seal 128 from contacting the shoulder surface between the large-diameter and small-diameter portions 120, 122 of the cylinder bore 118, at the entire area of the surface on the side of the output chamber 132. As shown in Fig. 10, a normally-open second solenoid-operated shut-off valve 140 is disposed in a portion of the rear brake cylinder passage 24 between the P valve 110 and the point of connection of the front and rear brake cylinder passages 22, 24. A reservoir passage 142 is connected at one end thereof to a portion of the rear brake cylinder passage 24 between the P valve 110 and the second shut-off valve 140, and at the other end to a reservoir 144. A normally-closed third solenoid-operated shut-off valve 146 is provided in the reservoir passage 142. In the present first embodiment, the first shut-off valve 100 constitutes an example of a master cylinder cut-off valve, and the second shut-off valve 140 constitutes an example of an intermediate valve while the third shut-off valve 146 constitutes an example of a pressure reducing valve. A pump passage 148 is connected at one end thereof to the reservoir 144 and at the other end to the rear brake cylinder passage 24. A pump 150 is provided in the pump passage 148, for sucking and pressurizing the brake fluid in the reservoir 144. The pump 150 is of a plunger type driven by a motor 152 to deliver the pressurized fluid in an intermittent manner. The output end or delivery end (the other end indicated above) of the reservoir passage 148 is connected to a portion of the rear brake cylinder passage 24 on the upstream side of the second shut-off valve 140, namely, on the side of the master cylinder 10. A return passage 154 is connected at one end thereof to a portion of the rear brake cylinder passage 24 between the P valve 110 and the second shut-off valve 140, and at the other end to a portion of the front brake cylinder passage 22 between the master cylinder 10 and the first shut-off valve 100. A check valve 156 is provided in the return passage 154. This check valve 156 inhibits a flow of the brake fluid in a direction from the master cylinder 10 toward the rear wheel brake cylinder 30, and permits a flow of the fluid in the reverse direction with the valve opening pressure difference being substantially zero. A check valve device 160 is disposed in a portion of the rear brake cylinder passage 24 between the point of connection of the rear brake cylinder passage 24 and the pump passage 148 and the point of connection of the front and rear brake cylinder passages 22, 24. The check valve device 160 includes a first check valve 162 whose opening pressure difference is not substantially zero, and a second check valve 164 whose opening pressure difference is substantially zero. The first and second check valves 162, 164 are disposed in parallel with each other and have opposite directions in which the brake fluid is permitted to flow. In the present embodiment, the pump 150 serves as the hydraulic pressure source when the wheel braking pressures are controlled in the anti-lock manner, and therefore the first check valve 162 is oriented so that the first or pressure reducing check valve 162 serves as a check valve which permits a flow of the brake fluid in the direction from the pump 150 toward the front wheel brake cylinder 20 after the pressure of the brake fluid delivered from the pump 150 becomes higher than the pressure in the front wheel brake cylinder 20 by more than a predetermined or preset opening pressure difference of the check valve 162. The principle of operation of the pump 150 as the hydraulic pressure source will be described later in detail. Referring next to Fig. 12, there will be explained flows of the brake fluid to and from the master cylinder 10, pump 150 and front and rear wheel brake cylinders 20, 30. It is noted that Fig. 12 schematically shows a major portion of the brake fluid circuit, while ignoring the provision of both of the P valve 110 and the normally-open second shut-off valve 140. During a normal operation of the present braking system without an operation of the pump 150, the brake fluid pressurized by the master cylinder 10 is supplied to the front wheel brake cylinder 20 through the first shut-off valve 100, and to the rear wheel brake cylinder 30 through the first shut-off valve 100 and the second check valve 164 of the check valve device 160. Since the opening pressure difference of the second check valve 166 is substantially zero, almost the same braking pressures are applied to the front and rear wheel brake cylinders 20, 30. When the pump 150 is operated, on the other hand, the first shut-off valve 100 is closed, and the brake fluid delivered from the pump 150 is supplied to the front wheel brake cylinder 220 through the first check valve 162, and supplied to the rear wheel brake cylinder 30 without flowing through the first check valve 162. Since the opening pressure difference of the first check valve 162 is not substantially zero, the braking pressure in the front wheel brake cylinder 20 is made lower than that in the rear wheel brake cylinder 30 by an amount corresponding to the preset opening pressure difference of the first check valve 162. The constructions of the first and second check valves 162, 164 will be described later in detail. The solenoids of the first, second and third solenoid-operated shut-off valves 100, 140 and 146 are connected to a controller 170, which is principally constituted by a computer, A/D converters and drivers. The computer incorporates a central processing unit (CPU), a read-only memory (ROM), a random-access memory (RAM) and a bus. The controller 170 selectively opens and closes the shut-off valves 100, 140, 146 as needed, depending upon the output signals of wheel speed sensors 172, 174, which represent the rotating speeds of the front and rear wheels. The motor 152 for driving the pump 150 is also controlled by the controller 170. In principle, the motor 152 and the pump 150 are turned off when the entire volume of the brake fluid in the reservoir 144 has been pumped up by the pump 150. While the motor 152 may be kept operated as long as the wheel braking pressures are controlled in the anti-lock manner, the pump 150 is stopped upon evacuation of the reservoir 144, to minimize the operating noise. The evacuation of the reservoir 144 may be detected directly by a position sensor (e.g., proximity switch) adapted to detect the axial position of a piston 176 of the reservoir 144, or indirectly by a load sensor adapted to detect a load acting on the motor 152 on the basis of an electric current applied to the motor 152, or by a timer adapted to measure the time of continuous operation of the motor 152. Where the evacuation is indirectly detected, the motor 152 is turned off when the detected load is lowered below a predetermined threshold value or when the measured operation time exceeds a predetermined limit. The controller 170 may be adapted to turn off the motor 152 and stop the pump 150 when it is required to discharge the brake fluid from both of the front and rear wheel brake cylinders 20, 30 for rapidly lowering the braking pressures during an anti-lock control of the braking pressures. In this respect, it is noted that an operation of the pump 150 to deliver the pressurized fluid to the rear brake cylinder passage 24 prevents reduction in the braking pressures in both of the front and rear wheel brake cylinders 20, 30. Further, the controller 170 may be adapted to stop the pump 150 when it is required to increase the braking pressures in the front and rear wheel brake cylinders 20, 30 by only a small amount. Since the pump 150 of the plunger type delivers the brake fluid intermittently at a certain cycle time, it is difficult to increase the braking pressures by an amount which is smaller than the amount of increase of the braking pressures by one delivery of the fluid from the pump 150. Therefore, when only a small amount of increase of the braking pressures is required, the pump 150 is stopped, and the first shut-off valve 100 is opened for a short length of time to permit the master cylinder pressure to be applied to the front and rear wheel brake cylinders 20, 30, for thereby raising the braking pressures by the desired small amount. There will next be described in detail an operation of the controller 170 for controlling the shut-off valves 100, 140 and 146. During braking of the vehicle, the controller 170 monitors the rotating conditions (e.g., deceleration values, slip amounts and slip ratios) of the individual wheels of the vehicle on the basis of the output signals of the wheel speed sensors 172, 174, and determine whether any wheels have a locking tendency. The controller 170 controls the shut-off valves 100, 140, 146 in a selected one of seven pressure control modes as indicated in TABLE 1 below, to control the wheel brake cylinders 20, 30. These seven pressure control modes are established by respective different combinations of the open and closed states of the three shut-off valves 100, 140, 146. Mode States of Shut-Off Valves Pressure Control States of Front and Rear Brake Cylinders 1 2 3 Front Rear 1 O O C M/C Increase M/C Increase 2 O C C M/C Increase Hold 3 O C O M/C Increase Reduction 4 C 0 C Pump Increase Pump Increase 5 C C C Pump Increase Hold 6 C C 0 Pump Increase Reduction 7 C O O Reduction Reduction To effect the anti-lock control of the wheel braking pressures, the controller 170 performs the following steps: (a) determining whether any one of the front and rear wheels of the two pressure application sub-systems has a locking tendency, and if it is determined that any wheel has a locking tendency, determining on the basis of the rotating condition of that wheel a pressure control command (selected from among a pressure reducing command, a pressure holding command and a pressure increasing command) which should be generated to control the braking pressure in the brake cylinder of the wheel in question; (b) then selecting one of the seven pressure control modes depending upon the determined pressure control command (pressure reducing, holding or increasing command) and depending upon whether the wheel having the locking tendency is a front or rear wheel; and (c) then controlling the pressure in the brake cylinder of the wheel in question in the selected pressure control mode. To this end, the ROM of the controller 170 stores routines for determining the pressure control commands for the individual wheels on the basis of the rotating conditions of the wheels, and routines for controlling (turning on or off) the solenoids of the respective shut-off valves 10,, 140, 146 according to the determined pressure control commands. The anti-lock pressure control operation of the present braking system will be described in detail, assuming that the front wheel associated with one of the two pressure application sub-systems has a locking tendency without a locking tendency of the rear wheel. In this case, the pressure in the front wheel brake cylinder 20 should be first reduced. However, the seven pressure control modes available do not include a mode for reducing only the pressure in the front wheel brake cylinder 20, as is apparent from TABLE 1. Therefore, the seventh pressure control mode is selected to reduce the pressures in both of the front and rear wheel brake cylinders 20, 30. In the seventh pressure control mode, the solenoid of the first shut-off valve 100 is turned ON to close this shut-off valve 100, so that the front and rear wheel brake cylinders 20, 30 are disconnected from the master cylinder 10. Further, the solenoid of the third shut-off valve 146 is turned ON to open this valve 146, so that the pressures in the front and rear wheel brake cylinders 20, 30 are reduced. Described more specifically, the front wheel brake cylinder 20 is brought into communication with the reservoir 144 through the second check valve 164, normally-open second shut-off valve 140 and now opened third shut-off valve 146, whereby the brake fluid is permitted to flow from the front wheel brake cylinder 20 to the reservoir 144. At the same time, the rear wheel brake cylinder 30 is communicated with the reservoir 144 through the P valve 110 and the opened third shut-off valve 146, and the brake fluid is permitted to flow from the rear wheel brake cylinder 30 to the reservoir 144. Thus, the braking pressures in both of the front and rear wheel brake cylinders 20, 30 are reduced in the seventh pressure control mode. The seventh pressure control mode of operation is terminated when the locking tendency of the front wheel is eliminated or considerably reduced as a result of the reduction in the wheel brake cylinders 20, 30. Then, the pressure in the front and rear wheel brake cylinders 20, 30 are controlled in a selected one of the fourth, fifth, sixth and seventh pressure control modes, depending upon the locking tendencies of the front and rear wheels. In the fourth pressure control mode, the first and third shut-off valves 100, 146 are both closed, while the second shut-off valve 140 is opened, so that the fluid delivered from the pump 150 is returned to the front wheel brake cylinder 20 through the first check valve 162, and to the rear wheel brake cylinder 30 through the opened second shut-off valve 140 and the P valve 110, whereby the pressures in the front and rear wheel brake cylinders 20, 30 are both increased. In this fourth pressure control mode, the pressure of the fluid delivered from the pump 150 is reduced by the first check valve 162 by the preset opening pressure difference of the check valve 162. Therefore, the braking pressure in the front wheel brake cylinder 20 is lower than the braking pressure in the rear wheel brake cylinder 30, by the opening pressure difference of the first check valve 162, when the braking pressures are increased. In the fifth pressure control mode, the three shut-off valves 100, 140, 146 are all closed, and the pressure in the front wheel brake cylinder 20 is increased by operation of the pump 150 as in the fourth mode, while the pressure in the rear wheel brake cylinder 30 is held constant. In the fifth pressure control mode, the brake fluid delivered from the pump 150 is not returned to the rear wheel brake cylinder 30, but is returned to only the front wheel brake cylinder 20. In the fourth mode, on the other hand, the brake fluid from the pump 150 is also returned to the rear wheel brake cylinder 30. Accordingly, the rate of increase in the pressure in the front wheel brake cylinder 20 is higher in the fifth mode than in the fourth mode, as indicated in Fig. 13. As also shown in this figure, the pressure in the rear wheel brake cylinder 30 is increased in the fourth mode while the pressure in the same cylinder is held constant in the fifth mode. In the sixth pressure control mode, the first and second shut-off valves 100, 140 are both closed while the third shut-off valve 146 is opened, whereby the pressure in the front wheel brake cylinder 20 is increased as in the fourth mode, while the pressure in the rear wheel brake cylinder 30 is reduced. In principle, the first, second and third pressure control modes are not used for the anti-lock control of the braking pressure of the front wheel brake cylinder 20. In these three modes, the first shut-off valve 100 is opened. During the anti-lock pressure control, it is desirable to disconnect the front and rear wheel brake cylinders 20, 30 from the master cylinder, in order to reduce the delivery pressure of the pump 150 and minimize the pressure pulsation of the fluid delivered from the pump 150. However, if it becomes necessary to increase the pressure in the front or rear wheel brake cylinder 20, 30 after the reservoir 144 is entirely evacuated with the entire volume of the fluid pumped up by the pump 150, an appropriate one of the first, second and third pressure control modes is established to increase the pressure in the wheel brake cylinder in question with the pressure generated by the master cylinder 10. When the pressure in the front wheel brake cylinder 20 is increased by operation of the pump 150 in the fourth or fifth pressure control mode, the check valve 104 functions as a pressure relief valve to prevent the front wheel braking pressure from exceeding the master cylinder pressure. While in the above the operation of the braking system upon occurrence of a locking tendency of the front wheel without a locking tendency of the rear wheel was described, there will be described next an operation upon occurrence of a locking tendency of the rear wheel without a locking tendency of the front wheel. In this case, it is necessary to first reduce the pressure in the rear wheel brake cylinder 30. To this end, the braking system is first placed in the third pressure control mode wherein the first and third shut-off valve 100, 146 are opened while the second shut-off valve 140 is closed, whereby substantially no anti-lock pressure control is effected with respect to the pressure in the front wheel brake cylinder 20. That is, the pressure in the front wheel brake cylinder 20 is increased by the pressure generated by the master cylinder 10, while the pressure in the rear wheel brake cylinder 30 is reduced through the opened third shut-off valve 146. Subsequently, the first through seventh pressure control modes are selectively established by the controller 170 as needed. While the front wheel does not have a locking tendency, the first, second and third pressure control modes are selectively established, and only the pressure in the rear wheel brake cylinder is controlled in the anti-lock manner. If the front wheel as well as the rear wheel has a locking tendency, or if only the front wheel has a locking tendency with the locking tendency of the rear wheel being eliminated, the front and rear wheel braking pressures or the front wheel braking pressure is/are controlled in the anti-lock manner as in the case where the front wheel has a locking tendency without a locking tendency of the rear wheel. The rear wheel has a locking tendency without a locking tendency of the front wheel if the front wheel lies on an area of an uneven friction-coefficient road surface which area has a relatively high friction coefficient, while the rear wheel lies on an area of the road surface having a relatively low friction coefficient. In this case, it is preferable to maximize the front wheel braking pressure while preventing the locking of the front wheel, so that the relatively high friction coefficient of the road surface area is utilized by the front wheel to reduce the braking distance of the vehicle. On the other hand, it is preferable to maximize the cornering force acting on the rear wheel, for improving the steering or directional stability of the vehicle. In other words, it is desired that the braking system be capable of increasing the front wheel braking pressure without increasing the rear wheel braking pressure, or capable of reducing the rear wheel braking pressure without increasing the front wheel braking pressure. In the present first embodiment, the fifth or sixth pressure control mode is established to increase the front wheel braking pressure without an increase in the rear wheel braking pressure, and the sixth pressure control mode is established to reduce the rear wheel braking pressure without a decrease in the front wheel braking pressure. Thus, the present embodiment assures not only reduction in the required braking distance of the vehicle but also an improvement of the steering stability of the vehicle in the case of braking of the vehicle while the front wheel is on the high friction-coefficient area of an uneven friction-coefficient road surface while the rear wheel is on the low friction-coefficient area. There will be described an advantageous effect of the anti-lock braking system according to the first embodiment, by reference to the graph of Fig. 14. In a normal braking of the vehicle initiated by depression of the brake pedal 14 by the vehicle driver, the master cylinder 10 rather than the pump 150 functions as the pressure source, and the master cylinder pressure is applied to the front wheel brake cylinder 20, irrespective of the existence of the check valve device 160. Accordingly, the front-rear force distribution point is moved from the zero point of the coordinate system of the graph of Fig. 14, along along a first basic distribution line and along a distribution line of the P valve 110 for the minimum-load run of the vehicle. If the vehicle is in the minimum-load run (one form of a vehicle run with a relatively small load), the force distribution point reaches a point "a" as indicated in Fig. 14, when the depression force acting on the brake pedal 14 has been increased to increase the front wheel braking pressure to a level slightly lower than a level at which the front wheel begins to be locked on the road surface. When the anti-lock control of the the front wheel braking pressure is started due to an excessive degree of locking of the front wheel as a result of a further increase in the depression force of the brake pedal 14, both the front wheel braking pressure and the rear wheel braking pressure are reduced in the seventh pressure control mode. Consequently, the force distribution point is moved from the point "a" in the left direction, as indicated in the graph of Fig. 14, to a point which lies on the first basic distribution line or the distribution line of the P valve 110 for the minimum-load run. In this specific example, the force distribution point is moved to point "b". The above explanation is based on an assumption that the brake pedal 14 is held depressed and the master cylinder pressure is continuously increased even after the first reduction of the front and rear wheel braking pressures is started, namely, an assumption that the delivery pressure of the pump 150 is increased from the level at the time of start of the first reduction of the wheel braking pressures. If the fourth pressure control mode of operation is initiated to increase the front and rear wheel braking pressures as a result of elimination of the locking tendency of the front wheel, the brake fluid delivered from the pump 150 is supplied to the front wheel brake cylinder 20, with the pressure reduction corresponding to the opening pressure differnce of the first check valve 162. However, the brake fluid delivered from the pump 150 is supplied to the rear wheel brake cylinder 30, without the pressure reduction. It is noted that some volume of the brake fluid has been stored in the reservoir 144 by the time the locking tendency of the front wheel has been eliminated, that is, when the fourth pressure control mode of operation is initiated. After the delivery of the brake fluid from the pump 150 is started, the front wheel braking pressure and force are held constant and only the rear wheel braking pressure and force are increased, until the first check valve 162 is opened. Accordingly, the force distribution point is moved from the point "b" in the positive direction along the vertical axis of the graph of Fig. 14 (along which the rear wheel braking force is taken), to a point which lies on a second basic distribution line or a distribution line of the P valve 110 for the full-load run. In this specific example, the pressure distribution point is moved to point "c". Subsequently, the force distribution point is moved from the point "c", along the distribution line of the P valve 110 for the full-load run, in the direction of increasing the rear wheel braking force, and eventually reaches point "d" of intersection between a rear wheel locking line for the minimum-load run and the distribution line of the P valve 110 for the full-load run. Then, the rear wheel braking pressure is controlled in the anti-lock manner so as to eliminate the locking tendency of the rear wheel. If the vehicle is in the full-load run (one form of a vehicle run with a relatively large load), the front-rear force distribution point is moved to point "e" as indicated in Fig. 14, when the depression force acting on the brake pedal 14 is increased to increase the front wheel braking force to a level slightly lower than the wheel locking level. A further increase in the brake pedal depression force will cause initiation of control of the front wheel braking pressure in the anti-lock manner. As a result, the force distribution point is moved to the point "b" indicated above. When the fourth pressure control mode of operation is initiated to increase the front and rear wheel braking pressures as a result of elimination of the locking tendency of the front wheel, the force distribution point is moved from the point "b" in the positive direction of the vertical axis to the point "c" as in the above case. With a further increase in the front and rear wheel braking pressures by operation of the pump 150, the force distribution point is further moved from the point "c" along the distribution line of the P valve 110 for the full-load run, and eventually reaches point "f" of intersection between the distribution line of the P valve 110 for the full-load run and a front wheel locking line for the full-load run. Thereafter, the anti-lock control is effected to eliminate the locking tendency of the front wheel. In the present embodiment, the actual distribution of the front and rear wheel braking forces is controlled, during the normal pressure control operation (without the anti-lock control of the braking pressures), according to a combination of the first basic distribution line and the distribution line of the P valve 110 for the minimum-load run, irrespective of whether the vehicle is in the minimum-load run or full-load run. During the anti-lock pressure control operation with the vehicle in the minimum-load run, on the other hand, the actual distribution of the front and rear wheel braking forces is controlled basically according to the ideal distribution curve for the minimum-load run, more precisely, according to a portion of the ideal distribution curve for the minimum-load run which is located below (in Fig. 14) the distribution line of the P valve 110 for the full-load run, and a portion of the distribution line of the P valve 110 for the full-load run which is located below the ideal distribution curve for the minimum-load run. During the anti-lock pressure control operation with the vehicle in the full-load run, the actual distribution is controlled basically according to the distribution line of the P valve for the full-load run, more precisely, according to a portion of the ideal distribution curve for the full-load run which is located below the distribution line of the P valve for the full-load run, and a portion of the distribution line of the P valve for the full-load run which is located below the ideal distribution curve for the full-load run. In the present embodiment, therefore, the sum of the front and rear wheel braking forces or the total wheel braking force during the full-load vehicle run is increased with respect to that in the conventional braking system in which the anti-lock pressure control operation is effected according to the first basic distribution line even during the full-load vehicle run. Accordingly, the required braking distance of the vehicle can be reduced in the present braking system. It will also be understood from the graph of Fig. 14 that the rear wheel braking pressure or force during the full-load vehicle run can be effectively increased to thereby reduce the required vehicle braking distance, even when the ideal rear braking force is smaller than the threshold level of the P valve 110, for example, when the fully-loaded vehicle is abruptly braked on a snow-covered road surface or other road surface having a low friction coefficient. In the present embodiment, the distribution of the front and rear wheel braking forces during the normal pressure control operation and during the anti-lock pressure control operation and the minimum-load vehicle run corresponds to the first distribution pattern, while the distribution of the front and rear wheel braking forces during the anti-lock pressure control operation and the full-load vehicle run corresponds to the second distribution pattern. The second distribution pattern defines the rear wheel braking force larger than that defined by the first distribution pattern, over the entire ranges of the front and rear wheel braking forces. In the present embodiment, the opening pressure difference of the first check valve 162 is determined or preset so that the second basic distribution line can intersect the rear wheel locking line which is determined for the full-load vehicle run on a road surface having a low friction coefficient (about 0.3), such as a road surface covered by compressed snow. In other words, the opening pressure difference of the first check valve 162 is determined to be relatively high, so that the rear wheel would be locked on such low friction-coefficient road surface upon excessive depression of the brake pedal 14 during the full-load vehicle run while the rear wheel braking pressure is lower than the threshold level of the P valve 110, if the rear wheel braking pressure was not controlled in the anti-lock manner. In practice, however, early locking of the rear wheel upon excessive depression of the brake pedal under such condition is prevented or restricted by the anti-lock pressure control operation. Namely, the present embodiment is adapted such that the anti-lock pressure control operation is effectively combined with the higher rear wheel braking pressure with respect to the front wheel braking pressure owing to the opening pressure difference of the first check valve 162, so that the actual front-rear force distribution pattern is sufficiently close to the ideal distribution curve for the full-load run, even while the rear wheel braking pressure is lower than the threshold level of the P valve 110. In this respect, too, the required vehicle braking distance can be significantly reduced during the full-load vehicle run. Thus, the anti-lock braking system according to the present embodiment is capable of reducing the required vehicle braking distance during the full-load run, not only during braking with a relatively large braking force on a relatively high friction-coefficient road surface, but also during braking with a relatively small braking force on a relatively low friction-coefficient road surface. For effectively reducing the required braking distance during the full-load vehicle run with a relatively large braking force, it is essential that the master cylinder pressure be increased even after the initiation of the anti-lock pressure control, so that the maximum amount of increase in the front and rear wheel braking pressures is sufficiently large. For this reason, the required braking distance during the full-load vehicle run with a relatively large braking force cannot always be effectively reduced. To effectively reduce the required braking distance during the full-load vehicle run with a relatively small braking force, on the other hand, the master cylinder pressure need not be increased even after the initiation of the anti-lock pressure control. Therefore, the required braking distance during the full-load vehicle run with a relatively small braking force can always be effectively reduced. Referring next to Fig. 15, there will be described in detail the construction of the check valve device 160. In the present embodiment, the check valve device 160 and the second shut-off valve 140 are constructed as a unit, as indicated in Fig. 10 by a square block of one-dot chain line, so that the required number of components and the overall size of the unit are reduced. The second shut-off valve 140 is provided in a housing 177. As well known in the art, the second shut-off valve 140 includes a solenoid 140a, and a drive member 140b in the form of a rod extending through the solenoid 140a in concentric relation with the solenoid 140a. The drive member 140b is supported by a stationary support member 140c in the form of a sleeve, such that the drive member 140b is slidably movable in the longitudinal direction relative of the support member 140c. The drive member 140b has a part-spherical upper end portion 140d (as seen in Fig. 15) which serves as a valve member, while a valve seat member 140e in the form of a sleeve is fixed concentrically to the support member 140c. The valve seat member 140e has an annular valve seat 140f at its end opposite to the valve member (upper end portion) 140d. The valve seat 140f cooperates with the valve member 140d to constitute a shut-off valve. The valve seat member 140e has a central communication passage 140g formed therethrough in the longitudinal direction. The communication passage 140g is open at the valve seat 140f. The housing 177 indicated above also has a passage 178 having a circular cross sectional shape for connecting the front wheel brake cylinder 20 and the rear wheel brake cylinder 30 (P valve 110). The valve seat member 140e is fixedly disposed in the passage 178 such that the valve seat member 140e is concentric with the passage 178 and such that there is left an annular gap between the valve seat member 140e and the circumferential surface of the passage 178. This annular gap functions as an annular passage 179 concentric with the central communication passage 140g formed through the valve seat member 140e. Within the annular passage 179, there are disposed a one-way sealing member in the form of a cup seal which serves as the second check valve 164, and a two-way sealing member in the form of an O-ring 140h. The cup seal (second check valve) and the O-ring 140h are arranged in the order of description in the direction from the front wheel brake cylinder 20 toward the rear wheel brake cylinder 30. These cup seal and O-ring 140i divide the annular passage 179 into a first portion on the side of the front wheel brake cylinder 20, a second portion on the side of the rear wheel brake cylinder 30, and a third intermediate portion 140i between the first and second portions. The intermediate portion 140i is connected to the output or delivery end of the pump passage 148. The valve seat member 140e has a passage 140j for fluid communication between the communication passage 140g and the annular passage 179. In a portion of the communication passage 140g between the point of connection to the passage 140j and the end of the passage 140g on the side of the front wheel brake cylinder 20, there is disposed the first check valve 162 in the form of a check valve with a spring-biased ball 140k. The ball 140k is normally held seated on a circular valve seat 140m (also formed on the valve seat member 140e) under a biasing action of biasing means in the form of a spring 1401. In the check valve device 160 constructed as described above, the first check valve 162 inhibits a flow of the brake fluid in the direction from the front wheel brake cylinder 20 toward the intermediate portion 140i of the annular passage 179. However, the second check valve 164 permits a flow of the brake fluid into the intermediate portion 140i with the opening pressure difference being substantially zero. Further, the first check valve 162 permits a flow of the brake fluid in the direction from the intermediate portion 140i toward the front wheel brake cylinder 20, when the pressure in the intermediate portion 140i is higher than the pressure in the front wheel brake cylinder 20 by more than a preset opening pressure difference of the first check valve 162. This opening pressure difference is determined by a biasing force of the spring 1401. The second check valve 164 always inhibits a flow of the fluid from the intermediate portion 140i toward the front wheel brake cylinder 20. The flows of the fluid between the intermediate portion 140i and the rear wheel brake cylinder 30 are controlled by the second shut-off valve 140 only. In Fig. 15, reference sign 140n denotes a spring as biasing means for biasing the valve member 140d in the direction away from the valve seat 140f. In the present check valve device 160, the central communication passage 140g in which the first check valve 162 is provided, and the annular passage 179 in which the second check valve 164 is provided are formed concentrically with each other, whereby the overall dimension of the check valve device 160 in the radial or diametric direction of the valve seat member 140e can be reduced to minimize the size of the braking system equipped with the check valve device 160. Further, the valve seat member 140e having the valve seats 140f and 140m also serves as means for defining the communication passage 140g and annular passage 179, and further functions to support the first and second check valves 162, 164. This arrangement is effective to minimize the number of the required components and the size of the braking system as a whole. It will be understood from the above description of the present embodiment that the check valve device 160, controller 170 and P valve 110 cooperate to constitute an example of the distribution control device for controlling the distribution of the braking forces to be applied to the front and rear wheels, according to a selected one of the first and second distribution patterns. It will also be understood that the passage 178 includes a portion which serves as a portion of the rear brake cylinder passage 24 between the point of connection to the front brake cylinder passage 24 and the point of connection to the pump passage 148, and that the central communication passage 140g and the annular passage 179 serve respectively as two concentric and mutually independent passages one of which has a circular cross sectional shape and the other of which has an annular cross sectional shape. It is further noted that the P valve 110 is disposed outside a hydraulic circuit which includes the first, second and third shut-off valves 100, 140, 146, reservoir 144 and pump 150. This arrangement makes it possible to manufacture the P valve 110 separately from an integral brake unit which includes the shut-off valves 100, 140, 146, reservoir 144 and pump 150 accommodated in a single common housing. Thus, the size and weight of the brake unit can be reduced. Where the P valve 110 and the brake unit are manufactured separately from each other, it is a common practice to install the brake unit within an engine room of the vehicle, more precisely, near the master cylinder 10, and locate the P valve at a portion of the vehicle body at which a conduit providing the rear brake cylinder passage 24 is supported, or near the brake which includes the rear wheel brake cylinder 30. Referring next to Figs. 16-19, there will be described a second embodiment of the present invention. The present embodiment is different from the first embodiment only in the manner in which the second shut-off valve 140 is controlled by the controller 170. In the fourth pressure control mode, the pressure in the front wheel brake cylinder 20 is relatively slowly increased while that in the rear wheel brake cylinder 30 is relatively rapidly increased, as indicated in Fig. 13. In this fourth mode, the rear wheel braking pressure is increased by operation of the pump 150. In this respect, the pump 150 does not deliver the pressurized brake fluid continuously but delivers the fluid intermittently, as indicated in the upper part of Fig. 19. Therefore, if the second shut-off valve 140 is held open in the fourth pressure control mode for a period longer than the delivery period of the pump 150, the entire amount of the fluid delivered by each delivery action of the pump 150 is supplied to the rear wheel brake cylinder 30, unless the first check valve 162 is opened by the delivery pressure of the pump 150. On the other hand, the diameter of the rear wheel brake cylinder 30 is usually smaller than that of the front wheel brake cylinder 20. Accordingly, when the same amount of the brake fluid is supplied to the front and rear wheel brake cylinders 20, 30, the pressure in the rear wheel brake cylinder 30 is more sensitively increased. Therefore, a continuous control of the wheel braking pressures in the fourth mode with the second shut-off valve 140 held open will result in an excessively rapid increase in the rear wheel braking pressure, leading to undesirable reduction in the control stability of the rear wheel braking pressure due to an overshoot of the pressure rise. In the light of the above drawback, the braking system according to the present second embodiment has a duty-cycle pressure control mode in which the second shut-off valve 140 is alternately turned on and off with a controlled duty cycle while the first and third shut-off valves 100, 146 are held closed. This duty-cycle pressure control mode is considered to be a compromise between the fourth mode (for slow increase of the front wheel braking pressure and rapid increase of the rear wheel braking pressure) and the fifth mode (for rapid increase of the front wheel braking pressure and holding of the rear wheel braking pressure). The duty cycle of the solenoid of the second shut-off valve 140 in the duty-cycle pressure control mode is not a fixed value but is variable for continuous change of the increase rates of the front and rear wheel braking pressures. In the duty-cycle pressure control mode, the tendency toward increasing the rear wheel braking pressure is increased if the characteristic of the fourth pressure control mode is dominant over that of the fifth pressure control mode, while the tendency toward increasing the front wheel braking pressure is increased if the characteristic of the fifth mode is dominant over that of the fourth mode. Therefore, the duty-cycle pressure control mode wherein the duty cycle of the second shut-off valve 140 can be continuously changed facilitates with high stability without an excessively high rate of increase in the rear wheel braking pressure, and assures adequate control of the distribution of the front and rear wheel braking pressures, namely, adequate control of distribution of the braking forces to be applied to the front and rear wheels. In the present second embodiment, the ROM of the controller 170 stores routines for controlling the second shut-off valve 140, as illustrated in the flow charts of Figs. 16-18. The flow chart of Fig. 16 illustrates the routine for controlling the solenoid of the second shut-off valve 140, and the flow chart of Fig. 17 illustrates a sub-routine executed in step S40 of the routine of Fig. 16, while the flow chart of Fig. 18 illustrates the routine for determining an OFF time T 1 of the solenoid. These routines will be first briefly explained. For assuring an increase in the pressure in the rear wheel brake cylinder 30 by operation of the pump 150, it is necessary to open the second shut-off valve 140 just when the pressurized brake fluid is delivered from the pump 150 and supplied to the rear wheel brake cylinder 30. In the present embodiment, the second shut-off valve 140 is open while its solenoid is held de-energized or OFF, namely, while the de-energization pulse is present. However, it is difficult to generate the de-energization pulses in synchronization with the intermittent delivery actions of the pump 150, respectively. The pump 150 has an operation cycle time consisting of the delivery time and the non-delivery time, which are substantially the same. The present embodiment is further adapted such that pairs of adjacent de-energization pulses are generated with a cycle time T 3 , as indicated in the lower part of Fig. 19. Each de-energization pulse has the width corresponding to the OFF time T 1 of the solenoid of the second shut-off valve 140 during which the valve 140 is held open. The two adjacent de-energization pulses have an interval T 2 which includes the solenoid OFF time T 1 , as also indicated in Fig. 19. This pulse interval T 2 is made equal to the delivery time of the pump 150, which is one half of the operation cycle time of the pump 150. According to this arrangement, the solenoid OFF time T 1 (open time of the shut-off valve 140) produced by one of the two de-energization pulses of each pair is usually held within the delivery time of the corresponding delivery time of the pump 150, even when the generation of the pair of de-energization pulses is not precisely timed with the delivery action of the pump 150 in its intermittent delivery operation. In this example shown in Fig. 19, the entirety of the OFF time T 1 of the former de-energization pulse occurs within the corresponding delivery time of the pump 150. However, the OFF times T 1 of the two de-energization pulses may partially overlap the corresponding delivery times of the pump 150. In this case, too, the total time during which the second shut-off valve 140 is open by the two pulses is equal to T 1 . Although it is possible that the de-energization pulse is generated for each delivery action or time of the pump 150, this arrangement is not desirable since the solenoid of the second shut-off valve 140 should be turned off and on for each delivery action of the pump 150, and the valve 140 should have a high response to the generation of the de-energization pulses. Further, this arrangement tends to cause a rapid increase in the rear wheel braking pressure. In view of these facts, the present embodiment is adapted such that pairs of de-energization pulses are generated with the cycle time T 3 which is two times the operation cycle time of the pump 150. That is, each pair of de-energization pulses is generated each time the pump 150 performs two adjacent delivery actions. Accordingly, during the cycle time t 3 , the pressurized brake fluid delivered from the pump 150 is supplied to the front wheel brake cylinder 20 for a time period T 3 - 2T 1 , and is supplied to the rear wheel brake cylinder 30 for a time period T 1 . Therefore, the ratio of the amounts of the fluid supplied from the pump 150 to the front and rear wheel brake cylinders 20, 30 is proportional to (T 3 - 2T 1 )/T 1 . As the ratio (T 3 - 2T 1 )/T 1 increases, the rate of increase in the front wheel braking pressure increases while the rate of increase in the rear wheel braking pressure decreases, whereby the braking force acting on the front wheel increases while the braking force acting on the rear wheel decreases. Thus, there exists a relationship between the ratio of the amounts of the brake fluid supply from the pump 150 to the front and rear wheel brake cylinders 20, 30, and the ratio of the braking pressures or forces of the front and rear wheels. Namely, the ratios of the front and rear wheel braking pressures and forces increase with an increase in the ratio of the amounts of the fluid supply from the pump 150 to the front and rear wheel brake cylinders 20, 30. Accordingly, the ratio of the braking forces of the front and rear wheels can be changed by changing the ratio of the amounts of the fluid supply from the pump 150 to the front and rear wheel brake cylinders 20, 30. The latter ratio can be changed by changing at least one of the solenoid OFF time T 1 (open time) of the second shut-off valve 140 and the cycle time T 3 at which the successive pairs of de-energization pulses are generated. If the solenoid OFF time T 1 is increased, for example, the ratio of the amount of fluid supply to the front wheel brake cylinder 20 to that to the rear wheel brake cylinder 30 is reduced, and the ratio of the braking force of the front wheel to that of the rear wheel is accordingly reduced. If the solenoid OFF time T 1 is reduced, the ratio of the amount of fluid supply to the front wheel brake cylinder 20 to that to the rear wheel brake cylinder 30 is increased, and the ratio of the braking force of the front wheel to that of the rear wheel is accordingly increased. If the cycle time T 3 is increased, the ratio of the fluid supply amounts of the front and rear wheel brake cylinders 20, 30 is increased, and the ratio of the braking forces of the front and rear wheels is accordingly increased. If the cycle time T 3 is reduced, the ratios of the fluid supply amounts of the front and rear wheel brake cylinders 20, 30 and the braking forces of the front and rear wheels are reduced. In the present second embodiment, only the solenoid OFF time T 1 of the second shut-off valve 140 is increased or reduced to continuously change the duty cycle of the shut-off valve 140, which is the ratio of the open time to the closed time of the shut-off valve 140. Further, in the present embodiment, the solenoid OFF time T 1 is determined on the basis of the numbers of reductions and increases of the front and rear wheel braking pressures. To this end, the controller 170 is provided with a pressure reduction counter CFR which is incremented when the front wheel braking pressure is reduced once, and decremented when the rear wheel braking pressure is reduced once. The content of this pressure reduction counter CFR indicates a relationship between the pressure reduction frequencies of the front and rear wheel brake cylinders 20, 30. The solenoid OFF time T 1 (width of each de-energization pulse) is increased by a predetermined constant value α each time the content of the counter CFR exceeds a positive threshold value +K, and decreased by the value α each time the content of the counter CFR becomes smaller than a negative threshold value -K. The OFF time T 1 is variable within a range between 0 and T 3 . If the reduction of the front wheel braking pressure has occurred relatively frequency (if the front wheel brake cylinder 20 has exhibited a relatively high tendency of pressure reduction), the content of the CFR exceeds the positive threshold value +K, and the OFF time or open time T 1 of the second shut-off valve 140 is increased, whereby the amount of the fluid supply from the pump 150 to the front wheel brake cylinder 20 is reduced to reduce the rate of increase in the front wheel braking pressure for thereby reducing the braking force applied to the front wheel. On the other hand, the amount of the fluid supply from the pump 150 to the rear wheel brake cylinder 30 is increased to increase the rate of increase in the rear wheel braking pressure for thereby increasing the braking force applied to the rear wheel. In this case, therefore, the braking function of the front wheel is reduced while that of the rear wheel is increased. If the reduction of the rear wheel braking pressure has occurred relatively frequently, the content of the CFR becomes smaller than the negative threshold value -K, and the OFF time or open time T 1 of the second shut-off valve 140 is reduced, whereby the amount of the fluid supply from the pump 150 to the front wheel brake cylinder 20 is increased to increase the rate of increase in the front wheel braking pressure for thereby increasing the braking force applied to the front wheel. On the other hand, the fluid supply from the pump 150 to the rear wheel brake cylinder 30 is inhibited to hold the rear wheel braking pressure at the present level for thereby maintaining the braking force presently acting on the rear wheel. In this case, therefore, the braking function of the front wheel is increased while an increase in the braking function of the rear wheel is inhibited. It will be understood that the present second embodiment is adapted such that the duty cycle of the solenoid of the third shut-off valve 140 is changed on the basis of at least one of the pressure reducing tendencies (pressure reducing hystereses) of the front and rear wheel brake cylinders 20, 30, by changing the solenoid OFF time T 1 (width of the de-energization pulse of the second shut-off valve 140) on the basis of the content of the pressure reducing counter CFR. Referring to the flow charts of Figs. 16 and 17, there will next be described in detail the routine for controlling the second shut-off valve 140. In this routine, the second shut-off valve 140 is controlled not only in the duty-cycle mode but also in the other pressure control modes, as explained below. The present routine of Fig. 16 for controlling the second shut-off valve 140 is executed at a predetermined time interval. The routine is initiated with step S10 to determine whether the braking system is in the process of the anti-lock pressure control. This determination is effected on the basis of flags provided in the RAM of the controller 170. If a negative decision (NO) is obtained in step S10, the control flow goes to step S60 in which a signal is generated to de-energized or turn OFF the solenoid of the second shut-off valve 140, to hold the valve 140 in the open state. Thus, one cycle of the present routine is terminated. If the anti-lock pressure control of the braking system is commenced during repetitive execution of the routine, an affirmative decision (YES) is obtained in step S10, and the control flow goes to step S20 to determine whether it is required to reduce the front wheel braking pressure. This determination is made on the basis of a flag provided in the RAM. If an affirmative decision (YES) is obtained step S20, the control flow goes to step S60 to turn OFF the solenoid of the second shut-off valve 140 for opening the valve 140. To reduce the front wheel braking pressure, the second shut-off valve 140 should be opened in the seventh pressure control mode as described above. In this seventh mode, the pressures in both of the front and rear wheel brake cylinders 20, 30 are reduced. If a negative decision (NO) is obtained in step S20, the control flow goes to step S30 to determine whether it is required to reduce the rear wheel braking pressure. This step S30 is provided to determine whether the pressure reduction is required for only the rear wheel brake cylinder 30. If an affirmative decision (YES) is obtained in step S30, the control flow goes to step S50 to energize or turn ON the solenoid of the shut-off valve 140 for closing the valve 140. In this case, only the rear wheel braking pressure is reduced regardless of the front wheel braking pressure. If no pressure reduction is required for not only the front wheel brake cylinder 20 but also the rear wheel brake cylinder 30, a negative decision (NO) is obtained in step S30, and step S40 is implemented to control the second shut-off valve 140 in the duty-cycle pressure control mode. In the preceding embodiment, the fourth or fifth pressure control mode is selected in this situation. In the present embodiment, the duty-cycle pressure control mode is selected rather than the fourth or fifth mode. In theory, the duty-cycle pressure control mode of operation is also performed in the first or second pressure control mode indicated in TABLE 1. In practice, however, the first and second modes are rarely selected during the anti-lock pressure control operation. In this sense, the duty-cycle pressure control mode is used as an alternative to the fourth or fifth mode. The sub-routine for controlling the second shut-off valve 140 in the duty-cycle pressure control mode is illustrated in detail in the flow chart of Fig. 17. In this sub-routine, step S100 is initially implemented to: read out the cycle time T 3 (which is a predetermined constant) from the ROM of the controller 170; read out from the RAM of the controller 170 a time lapse T from the start of the present cycle in which a pair of de-energization pulses each defining the solenoid OFF time T 1 is generated; and determine whether the time lapse T has reached the predetermined cycle time T 3 . Namely, step S100 is provided to determine whether the predetermined cycle time T 3 has passed. If a negative decision (NO) is obtained in step S100, step S110 is implemented to: read out the presently effective solenoid OFF time T 1 (from the RAM; and determine whether the time lapse T is shorter than the solenoid OFF time T 1 . The solenoid OFF time T 1 is determined by a routine illustrated in the flow chart of Fig. 18 as described below, and stored in the RAM. If an affirmative decision (YES) is obtained in step S110, the control flow goes to step S120 to turn OFF the solenoid of the shut-off valve 140 for opening the valve 140. Namely, the first one of the pair of de-energization pulses in question is generated. Then, step S130 is implemented to increment the time lapse T by a predetermined value ΔT. Thus, one cycle of execution of the sub-routine of Fig. 17 is terminated, and the control flow goes back to the main routine of Fig. 16. The sub-routine of Fig. 17 is executed each time step S40 of the main routine of Fig. 16 is executed. The following description refers to the situation where step S40 of the main routine of Fig. 16 or the sub-routine of Fig. 17 is repeatedly implemented without implementation of steps S50 and S60. When the time lapse T has reached the solenoid OFF time T 1 as a result of repeated execution of the sub-routine of Fig. 24, an affirmative decision (YES) is obtained in step S110, and the control flow goes to step S150 to read out the predetermined pulse interval T 2 from the ROM of the controller 170, and determine whether the time lapse T is shorter than the pulse interval T 2 . If an affirmative decision (YES) is obtained in step S150, the control flow goes to step S160 in which the solenoid of the second shut-off valve 140 is turned ON to close the valve 140. That is, the first de-energization pulse is terminated and replaced by an energization pulse which energizes or turn ON the solenoid. Step S160 is followed by step S130. If the time lapse T has reached pulse interval T 2 during the repeated execution of the sub-routine of Fig. 17, a negative decision (NO) is obtained in step S150, and the control flow goes to step S170 to determine whether the time lapse T is shorter than a sum of the solenoid OFF time T 1 and the pulse interval T 2 . If an affirmative decision (YES) is obtained in step S170, step S180 is implemented to turn OFF the solenoid of the shut-off valve 140 to open the valve 140. Namely, the second one of the pair of de-energization pulses in question is generated. Then, the control flow goes to step S130. If the time lapse T has reached the sum of (T 1 + T 2 ) during the repeated execution of the sub-routine of Fig. 17, a negative decision (NO) is obtained in step S170, and the control flow goes to step S190 to turn ON the solenoid of the shut-off valve 140 to close the valve 140. Thus, the second de-energization pulse is terminated. Then, the control flow goes to step S130. When the time lapse T increases to the predetermined cycle time T 3 as a result of the repeated execution of the sub-routine of Fig. 17, an affirmative decision (YES) is obtained in step S100, and step S140 is implemented to reset the time lapse T for starting the next cycle of generation of a pair of de-energization pulses. With the sub-routine of Fig. 17 executed repeatedly, the second shut-off valve 140 is controlled (opened and closed) by successive pairs of de-energization pulses which are generated periodically with the cycle time T 3 , such that the two de-energization pulses of each pair have the pulse interval T 2 , and each pulse has a width corresponding to the solenoid OFF time T 1 of the second shut-off valve 140, as indicated in the lower part of Fig. 19. The duty cycle of the second shut-off valve 140 is controlled by determining the solenoid OFF time T 1 according to the routine of Fig. 18, which will be described in detail. The routine of Fig. 18 is executed at a predetermined interval. The routine is initiated with step S200 to determine whether the braking system is in the process of the anti-lock pressure control. If a negative decision (NO) is obtained in step S200, the control flow goes to step S210 to reset the content of the pressure reduction counter CFR and flags FX and FY to zero, and also rest the solenoid OFF time T 1 to a predetermined initial value T 10 . The content of the counter CFR, the values of the flags FX, FY and the OFF time T 1 are stored in the RAM of the controller 170. The functions of the flags FX, FY will be described. One cycle of execution of the routine of Fig. 18 is terminated with step S210. If the anti-lock pressure control is commenced during repeated execution of the routine of Fig. 18, an affirmative decision (YES) is obtained in step S200, and the control flow goes to step S220 to determine whether it is required to reduce the front wheel braking pressure. If an affirmative decision (YES) is obtained in step S220, step S230 is implemented to determine whether the flag FX is set at "0". If an affirmative decision (YES) is obtained in step S230, the control flow goes to step S240 to increment the pressure reduction counter CFR, and then step S240 to set the flag FX to "1". Step S250 is followed by step S260 to determine whether the content of the counter CFR is larger than the positive threshold value +K. If an affirmative decision (YES) is obtained in step S260, step S310 is implemented to increase the solenoid OFF time T 1 by a predetermined value a, and store the increased OFF time T 1 in the RAM. Step S310 is followed by step S320 to reset the pressure reduction counter CFR to zero. Then, step S270 is implemented to determine whether the content of the counter CFR is smaller than the negative threshold value -K. If a negative decision (NO) is obtained in step S270, the present cycle of execution of the routine of Fig. 18 is terminated. If it is still required to reduce the front wheel braking pressure in the next cycle, an affirmative decision (YES) is obtained in step S220, and a negative decision (NO) is obtained in step S230 since the flag FX has been set at "1". Consequently, the control flow goes to step step S260 while skipping steps S240 and S250. Thus, the flag FX is provided to increment the pressure reduction counter CFR only once during repeated execution of the present routine while the reduction of the front wheel braking pressure is continuously required. In other words, the flag FX functions to increment the counter CFR only once for generation of each command to reduce the front wheel braking pressure. If the reduction of the front wheel braking pressure is no more required, a negative decision (NO) is obtained in step S220, and the control flow goes to step S280 to reset the flag FX to zero. Then, step S290 and the following steps S300, S330, S340 and S350 are implemented for decrementing the counter CFR. Steps S290, S300, S330, S340 and S350 for the rear wheel brake cylinder 30 are equivalent to steps S220, S280, S230, S240 and S250 for the front wheel brake cylinder 20. Then, step S260 is implemented to determine whether the content of the counter CFR is larger than the positive threshold value +K. If a negative decision (NO) is obtained in step S260, step S270 is implemented to determine whether the content of the counter CFR is smaller than the negative threshold value -K. If a negative decision (NO) is obtained in step S270, the present cycle of execution of the routine of Fig. 18 is terminated. If an affirmative decision (YES) is obtained in step S270, the control flow goes to step S360 to read out the solenoid OFF time T 1 from the RAM of the controller 170, and reduce the solenoid OFF time T 1 by the predetermined value a. Step S360 is followed by step S370 i which the counter CFR is reset to zero, and one cycle of execution of the routine of Fig. 18 is terminated. It will be understood from the above explanation of the present third embodiment that the check valve device 160 functions as a pressure reduction control device, which cooperates with the controller 170 and P valve 110 to constitute the distribution control device for controlling the distribution of the braking forces of the front and rear wheels, according to one of the first and second distribution patterns. It will also be understood that the portions of the controller 170 assigned to implement step S40 of the routine of Fig. 16 (namely, sub-routine of Fig. 17 for controlling the second shut-off valve 140 in the duty-cycle pressure control mode) and the routine of Fig. 18 for determining the solenoid OFF time T 1 function as means for changing the duty cycle of the shut-off valve 140 on the basis of the numbers of the pressure reductions which have been required for the wheel brake cylinders 20, 30. Referring next to Figs. 20 and 21, there will be described a third embodiment of the present invention. The present third embodiment is different from the first embodiment only in the construction of the P valve and the connection of the P valve to the pump 150. In the first embodiment of Fig. 10, the output or delivery end of the pump passage 148 is connected to a portion of the rear brake cylinder passage 24 which is upstream of the second shut-off valve 140 and the P valve 110, so that the brake fluid delivered from the pump 150 is supplied to the rear wheel brake cylinder 30 through the shut-off valve 140 and the P valve 110. In this arrangement, the braking pressure in the rear wheel brake cylinder 30 is influenced by the pressure reducing function of the P valve 110 even during the anti-lock pressure control operation. This arrangement does not permit the rear wheel braking pressure to be increased to a sufficiently high level. In view of this drawback, the present third embodiment uses a P valve 180 which is directly connected to the pump 150, as shown in Fig. 21. The construction of the P valve 180 will be described by reference to Fig. 21. Since the P valve 180 is basically similar to the P valve 110, only a difference of the P valve 180 from the P valve 110 will be described. The same reference numerals as used in Fig. 11 will be used to identify the corresponding components of the P valve 180, which will not be described in the interest of simplification. The P valve 180 is designed so that it does not function during the anti-lock pressure control operation. Namely, the P valve 180 is adapted to receive the delivery pressure of the pump 150 during the anti-lock pressure control, which inhibits the valve piston 124 from performing a pressure reducing function, that is, prevents the valve piston 124 from being seated on the cup seal 128. Described in detail, the small-diameter portion 122 of the valve piston 124 is not directly exposed to an air chamber 181, but an auxiliary piston 182 fixed to the small-diameter portion 122 is exposed to the air chamber 181. A two-way sealing member in the form of an O-ring 183 is fluid-tightly fitted on the small-diameter portion 122, while a one-way sealing member in the form of a cup seal 184 is fluid-tightly fitted on the auxiliary piston 182. The O-ring 183 and the cup seal 184 define a pump pressure chamber 185 therebetween. The pump pressure chamber 185 is connected to the delivery or output side of the pump 150 through a communication passage 187 formed through a plug 186, a communication passage 188 formed through the housing 112, and a fluid passage 189 which is connected to the pump passage 148 as shown in Fig. 10. The auxiliary piston 182 has a center blind hole 190 open on its end face and exposed to the air chamber 181, such that the blind hole 190 forms a part of the air chamber 181. In other words, the blind hole 190 is formed to increase the volume of the air chamber 181, so that the volume of the air chamber 181 can be easily reduced when the valve piston 124 operates to perform its pressure reducing function. Since the same valve piston 124 as used in the P valve of Fig. 11 is used in the present P valve 180, the small-diameter portion 122 has a central blind hole 191, but this blind hole 191 is not essential in the present embodiment. The auxiliary piston 182 is normally held in abutting contact with the valve piston 124 under a biasing action of biasing means in the form of a spring 192. The opening pressure difference of the P valve 180 is determined by the biasing forces of both of the springs 126 and 192. The O-ring 183 functions to inhibit not only a flow of the brake fluid in a direction from the input chamber 130 toward the pump pressure chamber 185, but also a flow of the brake fluid in the opposite direction. In the anti-lock braking system provided with the P valve 180 constructed as described above, the brake fluid delivered from the pump 150 to increase the rear wheel braking pressure during the anti-lock pressure control is supplied to the rear wheel brake cylinder 30 through the second shut-off valve 140 and the P valve 180. At this time, the P valve 180 does not function to reduce the pressure of the fluid received from the pump 150. Explained more particularly, the auxiliary piston 182 is moved in a direction (in the right direction as seen in Fig. 21) that causes reduction in the volume of the air chamber 181, whereby the auxiliary piston 182 is brought into contact with the bottom of the air chamber 181. At the same time, the valve piston 124 is moved in the opposite direction (in the left direction as seen in Fig. 21) so that the large-diameter portion 120 is brought into contact with the bottom of the housing 112. Although the shoulder surface of the bottomed valve piston 124 fluid-tightly contacts the cup seal 128, the protrusions provided on the cup seal 128 as described above with respect to the P valve 110 permit the flow of the fluid between the input and output chambers 130, 132. As a result, the P valve 180 simply functions as a part of the rear brake cylinder passage 24. Therefore, the braking pressure in the rear wheel brake cylinder 30 can be increased to a level at which the check valve 156 is opened, namely, to the pressure in the master cylinder 10. Thus, the rear wheel braking pressure can be increased to a sufficiently high level, and the friction coefficient of the road surface on which the rear wheel lies can be effectively used to brake the vehicle, even in the presence of the P valve 180. Referring to the graph of Fig. 22, there will be described an advantageous effect of the present third embodiment of the invention. When the depression force acting on the brake pedal 14 is increased from zero in a normal braking condition without an anti-lock pressure control, the front-rear force distribution point is moved from the zero point of the coordinate system of the graph of Fig. 22, along a first basic distribution line (which is determined by the basic braking arrangement without the shut-off valves 100, 140, 146 and the first check valve 162). If the depression force on the brake pedal 14 is further increased during the minimum-load run of the vehicle, the force distribution point is further moved to point "a" of intersection between a distribution line of the P valve 180 and a front wheel locking line for the minimum-load run, as indicated in Fig. 22. If the anti-lock control is commenced in this condition to reduce both of the front and rear wheel braking pressures in the seventh pressure control mode, the force distribution point is moved to point "b". If the fourth pressure control mode is established to reduce the front and rear wheel braking pressures as a result of elimination of the locking tendency of the front wheel, the front wheel braking pressure is held constant until the first check valve 172 is opened, and is then increased. Accordingly, the force distribution point is moved from the point "b" in a direction parallel to the vertical axis of the graph of Fig. 22, which direction causes an increase in the rear wheel braking force. Since the P valve 180 is disabled while the rear wheel braking force is increased by operation of the pump 150, the force distribution point is then moved to point "c" which lies on a rear wheel locking line for the minimum-load run, As a result, the anti-lock control of the rear wheel braking pressure is initiated to eliminate the locking tendency of the rear wheel. If the force distribution point is moved to point "d" indicated in Fig. 22 during a normal braking operation with the vehicle placed in the full-load run, the anti-lock control of the front wheel braking pressure is initiated, and the force distribution point is then moved to the point "b" as a result of the anti-lock pressure control. If the locking tendency of the front wheel is subsequently eliminated, the force distribution point is moved from the point "b", via the point "c", to point "e" lying on a second basic distribution line, since the P valve 180 is disabled. The second basic distribution line is determined by the basic braking arrangement which includes the check valve device 160 (first check valve 162) but does not include the P valve 180. Then, the force distribution point is moved along the second basic line, in a direction that causes the rear wheel braking pressure to increase. Eventually, the force distribution line reaches point "f" of intersection between the second basic distribution line and a front wheel locking line for the full-load run. Then, the front wheel braking pressure is controlled in the anti-lock manner to avoid the locking tendency of the front wheel. In the present third embodiment, the actual distribution of the front and rear wheel braking forces is controlled, during the normal pressure control operation (without the anti-lock control of the braking pressures), according to the distribution line of the P valve 180, irrespective of whether the vehicle is in the minimum-load run or full-load run. During the anti-lock pressure control operation with the vehicle in the minimum-load run, on the other hand, the actual distribution of the front and rear wheel braking forces is controlled according to a distribution pattern which is sufficiently similar and close to the ideal distribution curve for the minimum-load run. During the anti-lock pressure control operation with the vehicle in the full-load run, the actual distribution of the front and rear wheel braking forces is controlled according to a portion of the ideal distribution curve for the full-load run which is located below (in Fig. 22) the second basic distribution line, and a portion of the second basic distribution line which is located below the ideal distribution curve for the full-load run. In the present embodiment, therefore, the rear wheel braking pressure during the full-load vehicle run can be further increased, whereby the total braking force of the vehicle can be increased, and the required vehicle braking distance can be reduced. It will also be understood from the graph of Fig. 29 that the rear wheel braking pressure or force during the full-load vehicle run can be effectively increased to thereby reduce the required vehicle braking distance, even when the ideal rear braking force is smaller than the threshold level of the P valve 180. In the present embodiment, the distribution of the front and rear wheel braking forces during the normal pressure control operation and during the anti-lock pressure control operation and the minimum-load vehicle run corresponds to the first distribution pattern, while the distribution of the front and rear wheel braking forces during the anti-lock pressure control operation and the full-load vehicle run corresponds to the second distribution pattern. The second distribution pattern defines the rear wheel braking force larger than that defined by the first distribution pattern, over the entire range of the rear wheel braking force. Like the braking system according to the first embodiment, the anti-lock braking system according to the third embodiment is capable of reducing the required vehicle braking distance during the full-load run, not only during braking with a relatively large braking force on a relatively high friction-coefficient road surface, but also during braking with a relatively small braking force on a relatively low friction-coefficient road surface. As indicated above with respect to the first embodiment, the required braking distance during the full-load vehicle run with a relatively large braking force cannot always be effectively reduced, but the required braking distance during the full-load vehicle run with a relatively small braking force can always be effectively reduced. In the present third embodiment, the check valve device 160 functions as the pressure reduction control device, which cooperates with the controller 170 and P valve 180 to constitute the distribution control device for controlling the distribution of the braking forces of the front and rear wheels, according to a selected one of the first and second distribution patterns. Further, the portions of the controller 170 assigned to implement step S40 of Fig. 16 (sub-routine of Fig. 17) and the routine of Fig. 18 function as the means for changing the duty cycle of the shut-off valve 140 on the basis of the numbers of the pressure reductions which have been required for the front and rear wheel brake cylinders. A fourth embodiment of this invention will be described by reference to Fig. 23. Like the anti-lock braking system according to the preceding third embodiment, the anti-lock braking system according to the present embodiment is also designed to be able to increase the rear wheel braking pressure to the level of the master cylinder pressure during the anti-lock pressure control, even in the presence of the P valve. In the third embodiment of Fig. 20, the P valve 180 is located between the rear wheel brake cylinder 30 and the second shut-off valve 140 as in the first embodiment of Fig. 10, but the P valve 180 is provided with means for disabling its pressure reducing function. In the present embodiment of Fig. 23, the P valve 110 as used in the first embodiment is used, but this P valve 110 is located upstream of the check valve device 160. Described in detail, the P valve 110 constructed as shown in Fig. 18 is disposed in a portion of the rear brake cylinder passage 24 between the point of connection between the front and rear brake cylinder passages 22, 24 and the check valve device 160. Thus, the delivery or output end of the pump passage 148 is located downstream of the P valve 110, so that the brake fluid delivered from the pump 150 during the anti-lock pressure control is supplied to the rear wheel brake cylinder 30 without passing through the P valve 110. Accordingly, the rear wheel braking pressure which is increased by operation of the pump 150 is not influenced by the pressure reducing function of the P valve 110. In the present fifth embodiment in which the P valve 110 is disposed between the front wheel brake cylinder 20 and the pump 150, the brake fluid delivered from the pump 150 during the anti-lock pressure control is first supplied to the output chamber 132 of the P valve 110 as shown in Fig. 18. As a result, the output pressure of the P valve 110 is increased, and the valve piston 124 is brought into abutting contact with the bottom of the plug 193 which constitutes a part of the housing 112. Subsequently, only the output pressure of the P valve is increased while the input pressure is held constant. When the output pressure of the P valve 110 exceeds the input pressure, the one-way sealing portion 134 of the cup seal 128 is opened, and the brake fluid is permitted to flow in the direction from the output chamber 132 toward the input chamber 130, through a gap between the one-way sealing portion 134 and the cylinder bore 118, whereby the brake fluid delivered from the pump 150 is supplied to the front wheel brake cylinder 20 through the P valve 110. Thus, the front wheel braking pressure is increased by operation of the pump 150. In the present fifth embodiment, the check valve device 160 functions as the pressure reduction control device, which cooperates with the controller 170 and P valve 110 to constitute the distribution control device for controlling the distribution of the braking forces of the front and rear wheels, according to a selected one of the first and second distribution patterns. Further, the portions of the controller 170 assigned to implement the sub-routine of Fig. 24 and the routine of Fig. 25 function as the means for changing the duty cycle of the shut-off valve 140 on the basis of the numbers of the pressure reductions which have been required for the front and rear wheel brake cylinders. In the several embodiments described above in detail, the opening pressure difference of the first check valve 72, 162 is fixed. However, the opening pressure differnce may be variable so that the opening pressure difference increases with an increase in the vehicle load or in the friction coefficient of the road surface. While the present invention has been described in detail in its presently preferred embodiments by reference to the accompanying drawings, it is to be understood that the prevent invention may be embodied with various changes, modifications and improvements, which may occur to those skilled in the art, without departing from the scope of the invention defined in the following claims.

Claims: A braking system for braking a motor vehicle by operation of a front and a rear brake for a front and a rear wheel of the vehicle, respectively, comprising a distribution control device (110, 160, 170, 180, 210, 241, 242, 256) which is provided to control a distribution of a front and a rear wheel braking force which are produced by said front and rear brakes (20, 30, 202, 204), respectively, and which are applied to said front and rear wheels, said distribution control device controlling said distribution according to a selected one of a first distribution pattern and a second distribution pattern, each of said first and second distribution patterns representing said front and rear wheel braking force with respect to each other, wherein said rear wheel braking force defined by said second distribution pattern is larger than that defined by said first distribution pattern over the entire ranges of the front and rear wheel braking forces or within portions of said entire ranges between zero and respective predetermined values, characterized in that said braking system selectively effects an anti-lock braking control to prevent said front and rear wheels from being locked during the operation of said front and rear brakes by monitoring rotating conditions of said front and rear wheels, and in that said distribution control device comprises a selecting device for selecting, during a running condition of said vehicle in which a load acting on said vehicle is within a first range of said load which is higher than a second range of said load, said first distribution pattern when said anti-lock braking control is not effected, and said second distribution pattern when said anti-lock braking control is effected, said distribution control device controlling said distribution according to one of said first and second distribution patterns which is selected by said selecting device. A braking system according to claim 1 of a diagonal or X-crossing type for a four-wheel motor vehicle, having two pressure application sub-systems which are connected to respective two mutually independent pressurizing chambers of a master cylinder (10, 224), each of said two pressure application sub-systems including (a) a front brake cylinder passage (22, 226) connecting a corresponding one of said two pressurizing chambers of said master cylinder and a front wheel brake cylinder (20, 206) of said front brake, (b) a rear brake cylinder passage (24, 228) connecting said front brake cylinder passage and a rear wheel brake cylinder (30, 208) of said rear brake, (c) a master cylinder cut valve (100, 230) in the form of a normally-open shut-off valve disposed in a portion of said front brake cylinder passage between said master cylinder and a point of connection of said front and rear brake cylinder passages, (d) a reservoir passage (142, 234) connected at one of opposite ends thereof to said rear brake cylinder passage, (e) a reservoir (144, 232) connected to the other end of said reservoir passage, (f) a pressure reducing valve (146, 236) in the form of a normally-closed shut-off valve disposed in said reservoir passage, (g) a pump passage (148, 238) connected at one of opposite ends thereof to said reservoir and at the other end to at least one of said front and rear brake cylinder passages, (h) a pump (150, 240) disposed in said pump passage for delivering a working fluid from said reservoir to a portion of said each pressure application sub-system, and (i) a controller (170, 241) operable in an anti-lock pressure control mode for controlling said master cylinder cut valve, said pressure reducing valve and said pump to effect an anti-lock pressure control operation for controlling pressures of said fluid in said front and rear wheel brake cylinders in an anti-lock manner, and wherein said distribution control device comprises said controller, and a pressure reduction control device (160, 242) disposed in a portion of said each pressure application sub-system which is other than the portion between said master cylinder and the connection of said front and rear brake cylinder passages, said pressure reduction control device applying to said front wheel brake cylinder (20, 106) the pressure as generated by a first hydraulic pressure source in the form of said master cylinder, to thereby establish said first distribution pattern, when said controller is not placed in said anti-lock pressure control mode, said controller and said pressure reduction control device cooperating to establish said second distribution pattern such that the pressure generated by a second hydraulic pressure source which consists of at least one of said master cylinder and said pump is reduced by said pressure reduction control device and is then applied to said front wheel brake cylinder, when said controller is placed in said anti-lock pressure control mode. A braking system according to claim 1 of a diagonal or X-crossing type for a four-wheel motor vehicle, having two pressure application sub-systems which are connected to respective two mutually independent pressurizing chambers of a master cylinder (10, 224), each of said two pressure application sub-systems including (a) a front brake cylinder passage (22, 226) connecting a corresponding one of said two pressurizing chambers of said master cylinder and a front wheel brake cylinder (20, 206) of said front brake, (b) a rear brake cylinder passage (24, 228) connecting said front brake cylinder passage and a rear wheel brake cylinder (30, 208) of said rear brake, (c) a master cylinder cut valve (100, 230) in the form of a normally-open shut-off valve disposed in a portion of said front brake cylinder passage between said master cylinder and a point of connection of said front and rear brake cylinder passages, said master cylinder cut valve being closed when the braking system is in an anti-lock pressure control mode, and opened when the braking system is not in said anti-lock pressure control mode, (d) an intermediate valve (140, 254) in the form of a normally-open shut-off valve disposed in said rear brake cylinder passage (24, 228), (e) a reservoir passage (142, 234) connected at one of opposite ends thereof to a portion of said rear brake cylinder passage between said intermediate valve and said rear wheel brake cylinder, (f) a reservoir (144, 232) connected to the other end of said reservoir passage, (g) a pressure reducing valve (146, 236) in the form of a normally-closed shut-off valve disposed in said reservoir passage, (h) a pump passage (148, 238) connected at one of opposite ends thereof to said reservoir and at the other end to a portion of said rear brake cylinder passage between said intermediate valve and a point of connection of said front and rear brake cylinder passages, (i) a pump (150, 240) disposed in said pump passage for delivering a working fluid from said reservoir to a portion of said each pressure application sub-system, and (j) a controller (170, 241) operable in said anti-lock pressure control mode for controlling said master cylinder cut valve, said intermediate valve, said pressure reducing valve and said pump to effect an anti-lock pressure control operation for controlling pressures of said fluid in said front and rear wheel brake cylinders in an anti-lock manner, and wherein said distribution control device comprises said controller, and a check valve device (160, 256) disposed in a portion of said rear brake cylinder passage between the point of connection of said front and rear brake cylinder passages and a point of connection of said rear brake cylinder passage and said pump passage, said check valve device comprising a first check valve (162, 258) and a second check valve (164, 260), said first check valve permitting a flow of the fluid therethrough in a first direction from said pump toward said front wheel brake cylinder after the pressure generated by the pump becomes higher than the pressure in said front wheel brake cylinder by more than a predetermined difference, and inhibiting a flow of the fluid therethrough in a second direction opposite to said first direction, said second check valve permitting a flow of the fluid therethrough in said second direction and inhibiting a flow of the fluid therethrough in said first direction. A braking system according to claim 3, wherein at least a part of said portion of said rear brake cylinder passage (24, 228) between the point of connection of said front and rear brake cylinder passages (22, 226, 24, 228) and the point of connection of said rear brake cylinder passage (24, 228) and said pump passage (148, 238) consists of a first and a second passage (140g, 179) which are concentric with and mutually independent of each other and which have a circular and an annular cross sectional shape, respectively, said first check valve (162, 256) being disposed in one of said first and second passages while said second check valve (164, 260) being disposed in the other of said first and second passages. A braking system according to claim 3, wherein said distribution control device comprises (a) a proportioning valve (180) disposed in said rear brake cylinder passage (24, 228) and including a valve piston (124) which is operated to apply to said rear wheel brake cylinder (30, 208) the pressure as generated by said master cylinder (10, 224) or said pump (150, 240) as a hydraulic pressure source when the pressure as generated by said hydraulic pressure source is lower than a predetermined threshold level, and to reduce the pressure as generated by said hydraulic pressure source and apply the reduced pressure to said rear wheel brake cylinder (30, 208) after the pressure as generated by said hydraulic pressure source exceeds said predetermined threshold level, and (b) a disabling device (181, 182) for disabling said proportioning valve to prevent functioning of said proportioning valve to reduce the pressure to be applied to said rear wheel brake cylinder when the pressure in said rear wheel brake cylinder is increased. A braking system according to claim 5, wherein said disabling device comprises means (181, 182) for applying the pressure generated by said pump (150) to said valve piston (124) of said proportioning valve (180) in a direction opposite to a direction in which the valve piston is moved to reduce the pressure generated by said pump, whereby the proportioning valve is disabled when the pressure in said rear wheel brake cylinder is increased. A braking system according to claim 3, wherein said controller (170, 241) has a plurality of pressure control modes which are selectively established to control said master cylinder cut valve (100, 230), said intermediate valve (140, 254) and said pressure reducing valve (146, 236) in said anti-lock manner, said plurality of pressure control modes including (1) a mode in which said intermediate valve and said pressure reducing valve are both open while said master cylinder cut valve is closed, to reduce the pressures in both of said front and rear wheel brake cylinders (20, 30, 206, 208), (2) a mode in which said master cylinder cut valve and said intermediate valve are both closed while said pressure reducing valve is open, to increase the pressure in said front wheel brake cylinder by operation of said pump (150, 240), and reduce the pressure in said rear wheel brake cylinder, and (3) a duty-cycle pressure control mode in which said master cylinder cut valve and said pressure reducing valves are both closed while said intermediate valve is alternately closed and opened to increase the pressures in said front and rear wheel brake cylinders by operation of said pump. A braking system according to claim 7, wherein said controller (170, 241) comprises means for changing a duty cycle of the intermediate valve (140, 254) in said duty-cycle pressure control mode. A braking system according to claim 8, wherein said means for changing the duty cycle of said intermediate valve changes said duty cycle on the basis of at least one of pressure reducing tendency of said front wheel brake cylinder (20, 206) and pressure reducing tendency of said rear wheel brake cylinder (30, 208), which tendencies have been exhibited in said anti-lock pressure control operation under the control of said controller (170, 241). A braking system according to claim 1, wherein said front and rear brakes include, respectively, a front wheel brake cylinder (20, 206) and a real wheel brake cylinder (30, 208) which are supplied with a pressurized working fluid, and wherein said distribution control device selectively establishes said first and second distribution patterns of said front and rear wheel braking forces by controlling a front wheel braking pressure in said front brake cylinder and a rear wheel braking pressure in said rear wheel brake cylinder, and wherein said first distribution pattern of said front and rear wheel braking forces is established by a first distribution pattern of said front and rear wheel braking pressures in which said front and rear wheel braking pressures are equal to each other, while said second distribution pattern of said front and rear wheel braking forces is established by a second distribution pattern of said front and rear wheel braking pressures in which said front braking pressure is reduced with respect to said rear wheel braking pressure. A braking system according to claim 10, further comprising an electrically operated first pressure source (150, 240), and a second pressure source (10, 224) which is connected to said front and rear wheel brake cylinders through an electrically operated shut-off valve (100, 230), and wherein said distribution control device comprises a pressure reducing device (160, 242, 256) disposed between said shut-off valve and said rear wheel brake cylinder, said pressure reducing device including a first check valve (162, 258) which permits a flow of a working fluid in a first direction from said first pressure source toward said front wheel brake cylinder after the pressure generated by said first pressure source becomes higher than the pressure in said front wheel brake cylinder by more than a predetermined difference, and a second check valve (164, 260) which always permits a flow of the fluid in a second direction from said second pressure source toward said rear wheel brake cylinder, said first pressure source being connected to said rear wheel brake cylinder, said distribution control device further comprising a controller (170, 241) for controlling said first pressure source and said shut-off valve such that said shut-off valve is opened to apply a pressure generated by said second pressure source to said front and rear wheel brake cylinders, and such that said first pressure source is activated while said shut-off valve is closed, to apply a pressure generated by said first pressure source to said front and rear wheel brake cylinders. A braking system according to claim 1, wherein said distribution control device selects said second distribution pattern when the rear wheel braking force is being increased while said anti-lock braking control is effected.

Inventors: ITO KENJI, KADOWAKI YOSHINORI, KONDO TAKASHI, KUSHI NAOTO, SUGITANI TATSUO, UCHIDA KIYOYUKI, ITO, KENJI, KADOWAKI, YOSHINORI, KONDO, TAKASHI, KUSHI, NAOTO, UCHIDA, KIYOYUKI, ITO, KENJI, C/O TOYOTA JIDOSHA K.K., KADOWAKI, YOSHINORI, C/O TOYOTA JIDOSHA K.K., KONDO, TAKASHI, C/O TOYOTA JIDOSHA K.K., KUSHI, NAOTO, C/O TOYOTA JIDOSHA K.K., SUGITANI TATSUO, C/O TOYOTA JIDOSHA K.K., UCHIDA, KIYOYUKI, C/O TOYOTA JIDOSHA K.K.
Applicants: TOYOTA MOTOR CO LTD, TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1995
Main classification: B60T8
IPC codes: B60T8