Patent Publication Number: US-2019193695-A1

Title: Vehicle braking device

Description:
TECHNICAL FIELD 
     This invention relates to a vehicle braking device. 
     BACKGROUND ART 
     In a vehicle braking device, there is a by-wire type vehicle braking device in which the hydraulic pressure (master pressure) of the master chamber provided in the master cylinder is adjusted and/or controlled independently of the operation of the brake operating member. Further, in a generally used vehicle braking device, a common master chamber (in a case where a plurality of master chambers is provided, this case can be also referred to as one common master chamber, as long as these chambers are mechanically inter-connected to one another) is connected to a plurality of wheel cylinders. Further, in a vehicle braking device, an actuator is provided between the master chamber and the wheel cylinders. The actuator executes an ABS control depending on the vehicle running situation and in the pressure decreasing control during the ABS control, the fluid in the wheel cylinders returns to the master chamber side. Such vehicle braking device is shown in, for example, a patent publication No. JP 2015-143060 A. 
     CITATION LIST 
     Patent Literature 
     [Patent Literature 1] JP 2015-143060 A 
     SUMMARY OF INVENTION 
     Technical Problem(s) 
     However, according to the vehicle braking device as explained above, when the ABS control is executed at only some of the plurality of vehicle wheels, the fluid flows in or flows out of the common master chamber as a result of pressure decreasing control or the pressure increasing control during the ABS control and such ABS control transitionally influences on the wheel cylinders at which the ABS control is not executed. For example, during an ABS control executed at a first vehicle wheel, when the ABS control is shifted from the pressure decreasing control to the pressure increasing control, the pressure (wheel cylinder pressure) in the wheel cylinder corresponding to the first vehicle wheel may be possibly reduced to a value smaller than the wheel cylinder pressures in the other wheel cylinders of corresponding other vehicle wheels. Under such situation, a relatively large amount of fluid flows into the wheel cylinder of the first vehicle wheel from the common master chamber. Due to such flow of the fluid into the wheel cylinder of the first vehicle wheel, the master pressure drops temporarily to a value lower than the expected value, which may cause a delay of rising of braking force at the vehicle wheels other than the first vehicle wheel. In other words, this may create an imbalance of applied braking force between the first vehicle wheel and other vehicle wheels. Thus, the above explained conventional vehicle braking device has still a room for improvements with respect to the stability of vehicle behavior. 
     Particularly in a by-wire type vehicle braking device, the fluctuations of master pressure derived from the ABS control executed at some of the plurality of vehicle wheels is not transmitted to the brake operating member and accordingly, such fluctuations of the master pressure is not absorbed by the movement of the brake operating member. The fluctuations of the master pressure influence on the wheel cylinder pressure and eventually influence on the braking force. 
     The present invention was made in consideration with the above problems and the objective of the invention is to provide a vehicle braking device which suppress the fluctuations of master pressure derived from the ABS control executed only at some of the vehicle wheels. 
     Solution to Problem(s) 
     The vehicle braking device according to the invention is characterized in that the vehicle braking device is a by-wire type vehicle braking device which includes a master cylinder having a master piston and a master chamber which volume is changed in response to a movement of the master piston, a driving portion which drives the master piston to adjust a master pressure which is a pressure of the master chamber independently of an operation of a brake operating member, an actuator provided in a hydraulic passage connecting the master chamber and a plurality of wheel cylinders for adjusting a hydraulic pressure in each of the plurality of wheel cylinders and a control portion which controls the driving portion and the actuator, wherein the actuator is configured to flow out a fluid in a pressure decreasing subject wheel cylinder among the plurality of wheel cylinders to flow towards the master chamber upon pressure decreasing control for the pressure decreasing subject wheel cylinder under an ABS control execution when the ABS control is executed by the control portion; and under the ABS control being executed at only one or some of a plurality of vehicle wheels corresponding to the plurality of wheel cylinders, the control portion executes a first control which controls the driving portion such that if an out-flow liquid amount of the fluid per unit time from the master chamber is greater than a predetermined out-flow amount, an increasing amount of the master pressure per unit time under a pressure increasing control of the master pressure becomes great or a decreasing amount of the master pressure per unit time under a pressure decreasing control of the master pressure becomes small, compared to a case where the out-flow liquid amount is equal to or less than the predetermined out-flow amount and/or the control portion executes a second control which controls the driving portion such that if an in-flow liquid amount of the fluid per unit time into the master chamber is greater than a predetermined in-flow amount, the increasing amount of the master pressure per unit time under the pressure increasing control of the master pressure becomes small or the decreasing amount of the master pressure per unit time under the pressure decreasing control of the master pressure becomes great, compared to a case where the in-flow liquid amount is equal to or less than the predetermined in-flow amount. 
     Effect of Invention 
     According to the invention, for example, a transitional rising of the master pressure due to a circulation of the fluid to the master chamber side upon pressure decreasing control under the ABS control can be suppressed by suppressing the increase of the master pressure or by accelerating the decrease thereof by the execution of the second control. Further, according to the invention, for example, a transitional decrease of the master pressure due to an increase of in-flow fluid into the ABS control subject wheel cylinder can be suppressed by suppressing the decrease of the master pressure or by accelerating the increase thereof by the execution of the first control. Thus, according to the invention, the fluctuations of the master pressure derived from the ABS control for one or some of the vehicle wheels can be suppressed. 
    
    
     
       BRIEF EXPLANATION OF ATTACHED DRAWINGS 
         FIG. 1  is a structural view of a vehicle braking device according to an embodiment of the invention; 
         FIG. 2  is an explanatory view explaining a behavior of a vehicle; 
         FIG. 3  is a time chart explaining the first control and the second control of the embodiment: and 
         FIG. 4  is a flowchart explaining the first control and the second control of the embodiment. 
     
    
    
     EMBODIMENTS FOR IMPLEMENTING INVENTION 
     The invention of the vehicle device according to one embodiment of the invention adapted to a vehicle will be explained hereinafter with reference to the attached drawings. The vehicle is equipped with a vehicle braking device A which applies hydraulic pressure braking force directly to each vehicle wheel Wfl, Wfr, Wrl and Wrr (in some case, collectively referred to as vehicle wheel “W”, front wheel “Wf” and rear wheel “Wr”) to apply brakes to the vehicle. The vehicle of the embodiment is a hybrid type vehicle with front wheel drive and is equipped with a regeneration braking device B which generates a regeneration braking force at the front wheel Wf. The regeneration braking device B includes a generator B 1  (B 1  in  FIG. 1 ) provided at the drive shaft of the front wheel Wf. It is noted here that although no illustration is shown in the drawings, the regeneration braking device B includes a hybrid ECU, a battery and an inverter. The regeneration braking device B generates the regeneration braking force which is obtained by converting the kinetic energy of the vehicle to the electric energy and applies the regeneration braking force to the wheel W (here in this embodiment, front wheel Wf). The operation of such regeneration braking device is well known and the detail explanation thereof is omitted.
         (Overall Structure)       

     The vehicle braking device A includes a brake pedal  11 , a master cylinder  12 , a stroke simulator portion  13 , a reservoir  14 , a booster mechanism (corresponding to the “driving portion”)  15 , an actuator  16 , a brake ECU (corresponding to the “control portion”)  17  and wheel cylinders WC, as shown in  FIG. 1 . 
     The wheel cylinders WCfl, WCfr, WCrl and WCrr (hereinafter collectively referred to as wheel cylinder WC) restrict the rotation of the wheel W and are disposed in the respective calipers CL. The wheel cylinder WC serves as a braking force applying mechanism which applies braking force to the wheel W of the vehicle based on the pressure (brake hydraulic pressure) of the brake liquid (corresponding to “fluid”) from the actuator  16 . When the brake hydraulic pressure is supplied to the wheel cylinder WC, each piston (not shown) in each wheel cylinder WC pushes a pair of brake pads (not shown) which serves as friction members and squeezes a disc rotor DR which serves as a rotational member rotating unitary with the wheel W from both sides thereof to thereby restrict the rotation of the rotor DR. It is noted here that in this embodiment, a disc type brake device is used but a drum type brake device may be used. 
     The brake pedal  11  corresponds to the brake operating member and is connected to the stroke simulator portion  13  and the master cylinder  12  via an operation rod  11   a . A stroke sensor  11   c  which detects a brake pedal stroke (operating amount: hereinafter, in some cases, referred to simply as “stroke”) by depression of the brake pedal  11 , which is under a braking operation state, is provided in the vicinity of the brake pedal  11 . The stroke sensor  11   c  is connected to the brake ECU  17  and the detected signal (detection result) is outputted to the brake ECU  17 . 
     The master cylinder  12  supplies the actuator  16  with the brake liquid in response to the operating amount of the brake pedal  11  and the master cylinder  12  is formed by a cylinder body  12   a , an input piston  12   b , a first master piston  12   c  and a second master piston  12   d , etc. 
     The cylinder body  12   a  is formed in a substantially bottomed cylinder shape housing having a bottom surface closed. The cylinder body  12   a  includes therein a partition wall portion  12   a   2  which extends inwardly with a shape of flange at the inner peripheral portion. An inner circumferential surface of the partition wall portion  12   a   2  is provided with a through hole  12   a   3  at a central portion thereof, penetrating through the partition wall portion  12   a   2  in a front/rearward direction. The cylinder body  12   a  is provided with a first master piston  12   c  and a second master piston  12   d  at an inner peripheral portion thereof at a portion further front side than the partition wall portion  12   a   2 . The first master piston  12   c  and the second master piston  12   d  are provided to be liquid-tightly movable in an axial direction in the cylinder body  12   a.    
     The cylinder body  12   a  is provided with an input piston  12   b  at an inner peripheral portion thereof at a portion further rear side than the partition wall portion  12   a   2 . The input piston  12   b  is liquid-tightly movable in an axial direction in the cylinder body  12   a . The input piston  12   b  slidably moves within the cylinder body  12   a  in response to the operation of the brake pedal  11 . 
     The operation rod  11  a which is operable in association with the movement of the brake pedal  11  is connected to the input piston  12   b . The input piston  12   b  is biased in a direction where the volume of the first hydraulic pressure chamber R 3  becomes large, i.e., in a rearward direction (right direction as viewed in the drawing) by means of a compression spring  11   b . When the brake pedal  11  is depressed, the operation rod  11   a  advances forward overcoming the biasing force of the compression spring  11   b . By this advance movement of the operation rod  11   a , the input piston  12   b  advances in association with the movement of the operation rod  11   a . When the depression operation of the brake pedal  11  is released, the input piston  12   b  retreats by the biasing force of the compression spring  11   b  and is brought into contact with a restriction projecting portion  12   a   4  for positioning the input piston thereat. 
     The first master piston  12   c  includes a pressurizing cylindrical portion  12   c   1 , a flange portion  12   c   2  and a projecting portion  12   c   3  in order from the front side and these portions  12   c   1 ,  12   c   2  and  12   c   3  are formed integrally as a unit. The pressurizing cylindrical portion  12   c   1  is formed in a substantially bottomed cylinder shape having an opening at a front portion thereof and a bottom wall at a rear portion thereof. The pressurizing cylindrical portion  12   c   1  is liquid-tightly movably provided in the inner peripheral surface of the cylinder body  12   a . A coil spring-shaped biasing member  12   c   4  is provided in the inner space of the pressurizing cylindrical portion  12   c   1  between the first master piston  12   c  and the second master piston  12   d . The first master piston  12   c  is biased in a rearward direction by the coil spring  12   c   4 . In other words, the first master piston  12   c  is biased by the coil spring  12   c   4  in a rearward direction and is finally brought into contact with a restriction projecting portion  12   a   5  for positioning. This position is defined to be the initial position (position predetermined in advance) at the time the depression operation of the brake pedal  11  is released. 
     The flange portion  12   c   2  is formed to have a greater diameter than the diameter of the pressurizing cylindrical portion  12   c   1  and is liquid-tightly and slidably disposed on an inner peripheral surface of a large diameter portion  12   a   6  in the cylinder body  12   a . The projecting portion  12   c   3  is formed to have a smaller diameter than the diameter of the pressurizing cylindrical portion  12   c   1  and is slidably and liquid-tightly fitted in the through hole  12   a   3  of the partition wall portion  12   a   2 . The rear end of the projecting portion  12   c   3  projects into an inner space of the cylinder body  12   a , passing through the through hole  12   a   3  and is separated from the inner peripheral surface of the cylinder body  12   a . The rear end surface of the projecting portion  12   c   3  is separated from the bottom surface of the input piston  12   b  and the separation distance therebetween is formed to be variable. 
     The second master piston  12   d  is arranged in the cylinder body  12   a  at a front side of the first master piston  12   c . The second master piston  12   d  is formed in a substantially bottomed cylinder shape having an opening at a front portion thereof. A coil spring  12   d   1  which serves as a biasing member is disposed in the inner space of the second master piston  12   d  between the second piston  12   d  and an inner bottom surface of the cylinder body  12   a . The second master piston  12   d  is biased by the coil spring  12   d   1  in a rearward direction. In other words, the second master piston  12   d  is biased by the coil spring  12   d   1  towards a predetermined initial position. 
     The master cylinder  12  is formed by a first master chamber R 1 , a second master chamber R 2 , a first hydraulic pressure chamber R 3 , a second hydraulic pressure chamber R 4  and a servo chamber (hydraulic pressure chamber) R 5 . In the explanation, hereinafter, the first master chamber R 1  and the second master chamber R 2  may be collectively referred to as master chambers R 1  and R 2 . The first master chamber R 1  is defined by the inner peripheral surface of the cylinder body  12   a , the first master piston  12   c  (front side of the pressurizing cylindrical portion  12   c   1 ) and the second master piston  12   d . The first master chamber R 1  is connected to the reservoir  14  via the hydraulic passage  21  which is connected to the port PT 4 . Further, the first master chamber R 1  is connected to the hydraulic passage  40   a  (actuator  16 ) via the hydraulic passage  22  which is connected to the port PT 5 . 
     The second master chamber R 2  is defined by the inner peripheral surface of the cylinder body  12   a  and the front side of the second master piston  12   d . The second master chamber R 2  is connected to the reservoir  14  via the hydraulic passage  23  which is connected to the port PT 6 . Further, the second master chamber R 2  is connected to the hydraulic passage  50   a  (actuator  16 ) via the hydraulic passage  24  which is connected to the port PT 7 . 
     The first hydraulic pressure chamber R 3  is formed between the partition wall portion  12   a   2  and the input piston  12   b  and is defined by the inner peripheral surface of the cylinder body  12   a , the partition wall portion  12   a   2 , the projecting portion  12   c   3  of the first master piston  12   c  and the input piston  12   b . The second hydraulic pressure chamber R 4  is formed at the side of the pressurizing cylindrical portion  12   c   1  of the first master piston  12   c  and is defined by the inner peripheral surface of the large diameter portion  12   a   6  of the cylinder body  12   a , the pressurizing cylindrical portion  12   c   1  and the flange portion  12   c   2 . The first hydraulic pressure chamber R 3  is connected to the second hydraulic pressure chamber R 4  via the hydraulic passage  25  which is connected to the port PT 1  and the port PT 3 . 
     The servo chamber R 5  is formed between the partition wall portion  12   a   2  and the pressurizing cylindrical portion  12   c   1  of the first master piston  12   c  and is defined by the inner peripheral surface of the cylinder body  12   a , the partition wall portion  12   a   2 , the projecting portion  12   c   3  of the first master piston  12   c  and the pressurizing cylindrical portion  12   c   1 . The servo chamber R 5  is connected to the output chamber R 12  via the hydraulic passage  26  which is connected to the port PT 2 . 
     The pressure sensor  26   a  is a sensor that detects the servo pressure which is supplied to the servo chamber R 5  and is connected to the hydraulic passage  26 . The pressure sensor  26   a  sends the detection signal (detection result) to the brake ECU  17 . The servo pressure detected by the pressure sensor  26   a  is an actual value of the hydraulic pressure in the servo chamber R 5  and hereinafter this pressure is named as the actual servo pressure (actual hydraulic pressure). 
     The stroke simulator portion  13  is formed by the cylinder body  12   a , the input piston  12   b , the first hydraulic pressure chamber R 3  and a stroke simulator  13   a  which is in fluid communication with the first hydraulic pressure chamber R 3 . 
     The first hydraulic pressure chamber R 3  is in fluid communication with the stroke simulator  13   a  via the hydraulic passages  25  and  27  which are connected to the port PT 1 . It is noted that the first hydraulic pressure chamber R 3  is in fluid communication with the reservoir  14  via a connection passage (not shown). 
     The stroke simulator  13   a  generates a stroke (reaction force) which magnitude depends on the operation state of the brake pedal  11  at the brake pedal  11 . The stroke simulator  13   a  is formed by a cylindrical portion  13   a   1 , a piston portion  13   a   2 , a reaction force hydraulic pressure chamber  13   a   3  and a spring  13   a   4 . The piston portion  13   a   2  liquid-tightly slidably moves within the cylindrical portion  13   a   1  in response to the braking operation which is the operation by the brake pedal  11 . The reaction force hydraulic pressure chamber  13   a   3  is formed between the cylindrical portion  13   a   1  and the piston portion  13   a   2  and defined thereby. The reaction force hydraulic pressure chamber  13   a   3  is in fluid communication with the first hydraulic pressure chamber R 3  and the second hydraulic pressure chamber R 4  via the connected hydraulic passages  27  and  25 . The spring  13   a   4  biases the piston portion  13   a   2  in a direction where the volume of the reaction force hydraulic pressure chamber  13   a   3  decreases. 
     It is noted that the first electromagnetic valve  25   a  which is a normally closed type electromagnetic valve is disposed in the hydraulic passage  25 . The second electromagnetic valve  28   a  which is a normally open type electromagnetic valve is disposed in the hydraulic passage  28  which connects the hydraulic passage  25  and the reservoir  14 . When the first electromagnetic valve  25   a  is in a closed state, the fluid communication between the first and the second hydraulic pressure chambers R 3  and R 4  is interrupted. This fluid communication interruption keeps the constant separation distance between the input piston  12   b  and the first master piston  12   c  to allow the coordinative movement therebetween. Further, when the first electromagnetic valve  25   a  is in an open state, the fluid communication between the first hydraulic pressure chamber R 3  and the second hydraulic pressure chamber R 4  is established. Thus, the volume change of the first and the second hydraulic pressure chambers R 3  and R 4  caused by the advance and/or retreat movement of the first master piston  12   c  can be absorbed by the transfer of the brake liquid. 
     The pressure sensor  25   b  is a sensor that detects the reaction force hydraulic pressure in the second hydraulic pressure chamber R 4  and the first hydraulic pressure chamber R 3  and is connected to the hydraulic passage  25 . The pressure sensor  25   b  also serves as an operating force sensor which detects the operating force to the brake pedal  11  and has a mutual relationship with the operating amount of the brake pedal  11 . The pressure sensor  25   b  detects the pressure in the second hydraulic pressure chamber R 4  when the first electromagnetic valve  25   a  is in a closed state and also detects the pressure (or the reaction force hydraulic pressure) in the first hydraulic pressure chamber R 3  which establishes a fluid communication with the second hydraulic pressure chamber R 4  when the first electromagnetic valve  25   a  is in an open state. The pressure sensor  25   b  sends the detection signal (detection result) to the brake ECU  17 . 
     The booster mechanism  15  generates a servo pressure in response to the operating amount of the brake pedal  11 . The booster mechanism  15  is a hydraulic pressure generating device which outputs an output pressure (in this embodiment, the servo pressure) acted by the inputted input pressure (in this embodiment, the pilot pressure) and generates a response delay in which the change of the output pressure relative to the change of the input pressure is delayed at the initial stage of starting of the pressure increasing operation or the pressure decreasing operation when the output pressure is intended to be increasing or decreasing. The booster mechanism  15  includes a regulator  15   a  and a pressure supply device  15   b.    
     The regulator  15   a  is configured to have a cylinder body  15   a   1  and a spool  15   a   2  which slides in the cylinder body  15   a   1 . The pilot chamber R 11 , the output chamber R 12  and the third hydraulic pressure chamber R 13  are formed in the regulator  15   a.    
     The pilot chamber R 11  is defined by the cylinder body  15   a   1  and a front end surface of a second large diameter portion  15   a   2   b  of the spool  15   a   2 . The pilot chamber R 11  is connected to the pressure decreasing valve  15   b   6  and the pressure increasing valve  15   b   7  (hydraulic passage  31 ) which are connected to the port PT 11 . A restriction projecting portion  15   a   4  is provided on the inner peripheral surface of the cylinder body  15   a   1  to position the spool  15   a   2  by bringing the second large diameter portion  15   a   2   b  into contact with the restriction projecting portion  15   a   4 . 
     The output chamber R 12  is defined by the cylinder body  15   a   1  and the small diameter portion  15   a   2   c , the rear end surface of the second large diameter portion  15   a   2   b  and the front end surface of the first large diameter portion  15   a   2   a  of the spool  15   a   2 . The output chamber R 12  is connected to the servo chamber R 5  of the master cylinder  12  via the hydraulic passage  26  which is connected to the port PT 12  and the port PT 2 . Further, the output chamber R 12  is connectible with the accumulator  15   b   2  via the hydraulic passage  32  which is connected to the port PT 13 . 
     The third hydraulic pressure chamber R 13  is defined by the cylinder body  15   a   1  and the rear end surface of the first large diameter portion  15   a   2   a  of the spool  15   a   2 . The third hydraulic pressure chamber R 13  is connectible with the reservoir  15   b   1  via the hydraulic passage  33  which is connected to the port PT 14 . A spring  15   a   3 , which biases the third hydraulic pressure chamber R 13  in a direction where the volume of the third hydraulic pressure chamber R 13  increases, is disposed in the third hydraulic pressure chamber R 13 . 
     The spool  15   a   2  is formed by the first large diameter portion  15   a   2   a , the second large diameter portion  15   a   2   b  and the small diameter portion  15   a   2   c . The first large diameter portion  15   a   2   a  and the second large diameter portion  15   a   2   b  are configured to be liquid-tightly slidably movable within the cylinder body  15   a   1 . The small diameter portion  15   a   2   c  is formed between the first large diameter portion  15   a   2   a  and the second large diameter portion  15   a   2   b  and is formed integrally therewith as a unit. The small diameter portion  15   a   2   c  is formed to have a diameter smaller than the first large diameter portion  15   a   2   a  and the second large diameter portion  15   a   2   b . Further, a communication passage  15   a   5  which connects the output chamber R 12  and the third hydraulic pressure chamber R 13  is formed in the spool  15   a   2 . 
     The pressure supply device  15   b  also serves as a drive portion which drives the spool  15   a   2 . The pressure supply device  15   b  includes a reservoir  15   b   1  which is a low pressure source, an accumulator  15   b   2  which is a high pressure source that accumulates the brake liquid (corresponding to “fluid”), a pump  15   b   3  which pumps the brake liquid from the reservoir  15   b   1  into the accumulator  15   b   2  and an electric motor  15   b   4  which drives the pump  15   b   3 . The reservoir  15   b   1  is exposed to the atmospheric pressure and the hydraulic pressure in the reservoir  15   b   1  is the same level with the atmospheric pressure. The pressure in the low pressure source is lower than the pressure in the high pressure source. The pressure supply device  15   b  is provided with a pressure sensor  15   b   5  which detects the pressure of the brake liquid supplied from the accumulator  15   b   2  and outputs the detected result to the brake ECU  17 . 
     Further, the pressure supply device  15   b  is provided with a pressure decreasing valve  15   b   6  and the pressure increasing valve  15   b   7 . In more detail, the pressure decreasing valve  15   b   6  is a normally open type electromagnetic valve which opens under a non-energized state. The flow-rate of the pressure decreasing valve  15   b   6  is controlled by the instructions from the brake ECU  17 . One side of the pressure decreasing valve  15   b   6  is connected to the pilot chamber R 11  via the hydraulic passage  31  and the other side thereof is connected to the reservoir  15   b   1  via the hydraulic passage  34 . The pressure increasing valve  15   b   7  is a normally closed type electromagnetic valve which closes under a non-energized state. The flow-rate of the pressure increasing valve  15   b   7  is controlled by the instructions from the brake ECU  17 . One side of the pressure increasing valve  15   b   7  is connected to the pilot chamber R 11  via the hydraulic passage  31  and the other side thereof is connected to the accumulator  15   b   2  via the hydraulic passage  35  and the hydraulic passage  32  which is connected to the hydraulic passage  35 . 
     The operation of the regulator  15   a  will be explained briefly hereinafter. In the case where the pilot pressure is not supplied to the pilot chamber R 11  from the pressure decreasing valve  15   b   6  and the pressure increasing valve  15   b   7 , the spool  15   a   2  is positioned at the initial position by means of a biasing force of the spring  15   a   3  (the state of  FIG. 1 ). The initial position of the spool  15   a   2  is determined to a position to be fixed by the contact of the front end surface of the spool  15   a   2  with the restriction projecting portion  15   a   4 . This initial position indicates the position immediately before the rear end surface of the spool  15   a   2  closes the port PT 14 . 
     As explained, when the spool  15   a   2  is in the initial position, the port PT 14  and the port PT 12  are in fluid communication with each other through the communication passage  15   a   5  and at the same time the port PT 13  is closed by the spool  15   a   2 . 
     In the case where the pilot pressure, which has been established in response to the brake pedal  11  operating amount by the operation of the pressure decreasing valve  15   b   6  and the pressure increasing valve  15   b   7 , increases, the spool  15   a   2  moves in a rearward direction (right side in  FIG. 1 ), overcoming the biasing force of the spring  15   a   3 . The spool  15   a   2  moves to the position where the port PT 13 , which has been closed by the spool  15   a   2 , opens. The port PT 14  which has been in the open state, is closed by the spool  15   a   2 . The position of the spool  15   a   2  under this state is defined to be the “pressure increasing position”. Under this state, the port PT  13  and the port PT  12  are in fluid communication with each other through the output chamber R 12  (Pressure increasing operation). 
     By balancing the force between the pushing force at the front end surface of the second large diameter portion  15   a   2   b   2  of the spool  15   a   2  and a force corresponding to the servo pressure, the positioning of the spool  15   a   2  is determined. This position of the spool  15   a   2  is defined to be the “holding position”. At the holding position, the port PT 13  and the port PT 14  are closed by the spool  15   a   2 . (Holding operation). 
     In the case where the pilot pressure which has been established in response to the brake pedal  11  operating amount by the operation of the pressure decreasing valve  15   b   6  and the pressure increasing valve  15   b   7 , decreases, the spool  15   a   2  which has been in the holding position, now moves in a frontward direction by the biasing force of the spring  15   a   3 . Then, the port PT 13  which has been in the closed state by the spool  15   a   2  keeps the closed state. The port PT 14  which has been in the closed state is open. The position of the spool  15   a   2  at this state is defined to be the “pressure decreasing position”. Under this state, the port PT 14  and the port PT 12  are in fluid communication with each other through the communication passage  15   a   5  (Pressure decreasing operation). 
     The above explained booster mechanism  15  establishes a pilot pressure in response to a stroke of the brake pedal  11  by the pressure decreasing valve  15   b   6  and the pressure increasing valve  15   b   7  and generates a servo pressure which responds to the stroke of the brake pedal  11  by the pilot pressure. The established servo pressure is supplied to the servo chamber R 5  of the master cylinder  12  and the master cylinder  12  supplies the wheel cylinder WC with the master pressure generated in response to the stroke of the brake pedal  11 . The pressure decreasing valve  15   b   6  and the pressure increasing valve  15   b   7  form a valve portion which adjusts the in-flow and out-flow of the brake liquid into or out of the servo chamber R 5 . 
     As explained, the vehicle braking device A according to the embodiment is formed by a by-wire type braking device. In other words, the vehicle braking device A is formed such that the adjustment of the master pressure can be performed independently of the operation of the brake pedal  11  (brake operating member) and the fluctuations of the master pressure do not directly influence on the brake pedal  11 . In other words, the vehicle braking device A is configured such that under a normal operation state, excluding the case of electric failure, the brake pedal  11  is not structured to directly push the first master piston  12   c.    
     The actuator  16  is a device which adjusts the brake hydraulic pressure to be applied to each wheel cylinder WC and a first conduit system  40  and a second conduit system  50  are provided. 
     The first conduit system  40  controls the brake hydraulic pressure to be applied to the front right wheel Wfr and the rear left wheel MI and the second conduit system  50  controls the brake hydraulic pressure applied to the front left wheel Wfl and the rear right wheel Wrr. In other words, the conduit system of this embodiment is an X-conduit (diagonal) system. 
     The hydraulic pressure supplied from the master cylinder  12  is transmitted to the respective wheel cylinders WC through the first and the second conduit systems  40  and  50 . In the first conduit system  40 , the hydraulic passage  40   a  is provided which connects the hydraulic passage  22  and the wheel cylinders WCfr and WCrl. In the second conduit system  50 , the hydraulic passage  50   a  is provided which connects the hydraulic passage  24  and the wheel cylinders WCfl and WCrr. Through these hydraulic passages  40   a  and  50   a , the hydraulic pressure supplied from the master cylinder  12  is transmitted to the wheel cylinders WC. 
     Each of the hydraulic passages  40   a  and  50   a  is branched to two passages,  40   a   1  and  40   a   2  and  50   a   1  and  50   a   2 , respectively. In the branched hydraulic passages  40   a   1  and  50   a   1 , the first pressure increasing control valves  41  and  51  which control increasing of the brake hydraulic pressure to the wheel cylinders WCfr and WCfl are disposed respectively and in the branched hydraulic passages  40   a   2  and  50   a   2 , the second pressure increasing control valves  42  and  52  which control increasing of the brake hydraulic pressure to the wheel cylinders WCrl and WCrr, are disposed respectively. 
     These first pressure increasing control valves and the second pressure increasing control valves  41 ,  42 ,  51 ,  52  are formed by a two-position electromagnetic valve or a pressure differential control valve (linear valve) which can control the valve state to be a fluid communication state and an fluid interrupted state. The first pressure increasing control valves and the second pressure increasing control valves  41 ,  42 ,  51 ,  52  are formed as a normally open type valve which controls the valve state such that when the control current to the solenoid coil provided in the first pressure increasing control valves and the second pressure increasing control valves  41 ,  42 ,  51 ,  52  is zero value (non-energized state), the valve becomes in a fluid communication state and when the control current to the solenoid coil flows (energized state), the valve becomes in a fluid interrupted state. The master chambers R 1  and R 2  are connected to the wheel cylinders WC by means of the hydraulic passages  22 ,  24 ,  40   a  and  50   a  (corresponding to “hydraulic pressure passage”). 
     The hydraulic passage portion in each of the hydraulic passages  40   a ,  50   a  between the first and the second pressure increasing control valves  41 ,  42 ,  51 ,  52  and each wheel cylinder WC is connected to the reservoirs  43 ,  53  via the hydraulic passages  40   b ,  50   b  as the pressure decreasing hydraulic passages. In the hydraulic passage  40   b , the pressure decreasing control valves  44 ,  45  formed by a two-position electromagnetic valve or a pressure differential control valve (linear valve) which controls the fluid communication state and fluid interrupted state are provided. Similarly, in the hydraulic passage  50   b , the pressure decreasing control valves  54 ,  55  formed by a two-position electromagnetic valve or a pressure differential control valve (linear valve) which controls the fluid communication state and fluid interrupted state are provided. The pressure decreasing control valves  44  is disposed between the first pressure increasing control valve  41  and the reservoir  43 . The pressure decreasing control valve  45  is disposed between the second pressure increasing control valve  42  and the reservoir  43 . The pressure decreasing control valve  54  is disposed between the first pressure increasing control valve  51  and the reservoir  53 . The pressure decreasing control valve  55  is disposed between the second pressure increasing control valve  52  and the reservoir  53 . These pressure decreasing control valves  44 ,  45 ,  54 ,  55  are the normally closed type electromagnetic valves which become a fluid interrupted state when the control current to the solenoid coil provided in the respective pressure decreasing control valves is zero value (non-energized state) and become a fluid communication stat when the control current to the solenoid coil flows (energized state). 
     The hydraulic passages  40   c  and  50   c , which are the return hydraulic passages, are provided between the reservoirs  43 ,  53  and the hydraulic passages  40   a  and  50   a  which are the main hydraulic passages. In the return hydraulic passages  40   c  and  50   c , the pumps  46  and  56  are disposed which suction and/or discharge the brake liquid from the reservoirs  43 ,  53  side towards the master cylinder  12  side or towards the wheel cylinder WC side. The pump  46  discharges the brake liquid towards hydraulic passage  40   a  at the upstream side of the pressure increasing control valves  41 ,  42  (towards the master chamber R 1  side). The pump  56  discharges the brake liquid towards the hydraulic passage  50   a  at the upstream side of the pressure increasing control valves  51 ,  52  (towards the master chamber R 2  side). The pumps  46 ,  56  are driven by the motor  47 . The pumps  46 ,  56  suction the brake liquid from the reservoirs  43 ,  53  and discharges the same to the hydraulic passages  40   a ,  50   a  thereby to supply (return) the master chambers R 1  and R 2  side with the brake liquid. In other words, the pumps  46 ,  56  pump up the brake liquid from the wheel cylinders WC to the master chambers R 1  and R 2  by driving. 
     The brake ECU  17  is structured such that the detection signals from the wheel speed sensor S which is provided at the vehicle wheel W. The brake ECU  17  calculates the wheel speed of each wheel W, a presumed vehicle speed and the slip ratio, etc., based on the detection signal from the wheel speed sensor S. The brake ECU  17  executes the ABS control (anti-skid control) based on the calculation result. It is noted that the target servo pressure (target master pressure) set in response to the brake operation or under various circumstances have a dead zone which has a certain width band. 
     Various controls using the actuator  16  are executed by the instructions from the brake ECU  17 . For example, the brake ECU  17  outputs the control current that controls the various control valves  41 ,  42 ,  44 ,  45 ,  51 ,  52 ,  54  and  55  and the motor  47  which drives pumps provided in the actuator  16  to control the hydraulic pressure circuit in the actuator  16  to thereby independently control the wheel cylinder pressure which is the pressure in the wheel cylinder WC. The brake ECU  17  performs the ABS control which prevents the wheels from locking upon wheel being slipping, or about to be slipping during braking operation by controlling the actuator  16  to decrease, hold or increase the wheel cylinder pressure. The actuator  16  may be said to correspond to an ABS system (Anti-lock Brake System). 
     An example of the ABS control will be explained hereinafter, for example, in a case of controlling of the front right wheel Wfr. Under the pressure decreasing control in the ABS control, the first pressure increasing control valve  41  is controlled to be in a closed state and the pressure decreasing control valve  44  is controlled to be in an open state to thereby control the pump  46  to be driven. Then, the brake liquid in the wheel cylinder WCfr is introduced into the reservoir  43  through the pressure decreasing control valve  44  and the brake liquid in the reservoir  43  flows out to the upstream side (first master chamber R 1  side) of the first pressure increasing control valve  41  through the pump  46 . Since the first pressure increasing control valve  41  is in the closed state, the brake liquid pumped out from the pump  46  does not flow to the wheel cylinder WCfr side and accordingly, influences on the master pressure. 
     On the other hand, Under the pressure increasing control in the ABS control, the first pressure increasing control valve  41  is controlled to be in an open state (or in a differential pressure generating state: in a throttled state) and the pressure decreasing control valve  44  is controlled to be in a closed state. Under a holding control in the ABS control, both the first pressure increasing control  41  and the pressure decreasing control valve  44  are controlled to be in the closed state. The state that the ABS is operating is the state that the ABS control is being executed. 
     In summary, the vehicle braking device A is a by-wire type vehicle braking device which includes a master cylinder  12  which includes master pistons  12   c  and  12   d  and master chambers R 1  and R 2  the volumes of which are changeable in response to a movement of the master pistons  12   c  and  12   d , a booster mechanism (driving portion)  15  which adjusts the master pressure which corresponds to the pressures in the master chambers R 1  and R 2  by driving the master pistons  12   c  and  12   d , independently of the operation of the brake pedal (brake operating member)  11 , an actuator  16  provided in a hydraulic passage (hydraulic pressure passage)  22 ,  24 ,  40   a  and  50   a  which connects the master chambers R 1  and R 2  and a plurality of wheel cylinders WC and adjusts a hydraulic pressure in each wheel cylinder WC and a brake ECU (control portion)  17  which controls the booster mechanism  15  and the actuator  16 . The actuator  16  is configured to flow out the fluid towards the master chambers R 1  and R 2  side from a or some of the wheel cylinders WC which is the subject wheel cylinder for pressure decreasing operation of the wheel cylinder WC under the ABS control, when the ABS control is executed by the brake ECU  17 .
         (First Control and Second Control)       

     It is noted here that the brake ECU  17  is configured to (set to) execute the first control and the second control under a certain condition. The brake ECU  17  executes either the first control or the second control depending on the situation of ABS control of one or some of the wheels W among the plurality of wheels corresponding to the plurality of wheel cylinders WC. 
     The “first control” is a control which controls the booster mechanism  15  such that when the out-flow liquid amount (cc/s) of the brake liquid per unit time from the master chambers R 1  and R 2  is greater than a predetermined out-flow amount, comparing to a case where the out-flow liquid amount is equal to or less than the predetermined out-flow amount, the increasing amount of the master pressure per unit time during the pressure increasing operation of the master pressure becomes large or the decreasing amount of the master pressure per unit time during the pressure decreasing operation of the master pressure becomes small. The out-flow liquid amount can be said to be a flow-rate of the fluid out-flowing from the master chambers R 1  and R 2  to the actuator  16 . 
     Further, the “second control” is a control which controls the booster mechanism  15  such that when the in-flow liquid amount (cc/s) of the brake liquid per unit time to the master chambers R 1  and R 2  is greater than a predetermined in-flow amount, comparing to a case where the in-flow liquid amount is equal to or less than the predetermined in-flow amount, the increasing amount of the master pressure per unit time during the pressure increasing operation of the master pressure becomes small or the decreasing amount of the master pressure per unit time during the pressure decreasing operation of the master pressure becomes large. The in-flow liquid amount can be said to be a flow-rate of the fluid in-flowing to the master chambers R 1  and R 2  from the actuator  16 . 
     The first control can be said to be a control executed under a state that the ABS control is executed for only one or some of the wheels W and that the brake ECU  17  judges that the out-flow liquid amount of fluid from the master chambers R 1  and R 2  is larger than the predetermined out-flow liquid amount. Further, the second control can be said to be a control executed under a state that the ABS control is executed for only one or some of the wheels W and that the brake ECU  17  judges that the in-flow liquid amount of fluid to the master chambers R 1  and R 2  is larger than the predetermined in-flow amount. The brake ECU  17  may be said to include a judging portion which judges the magnitude relation of the flow amount. 
     The judgement explained above will be explained more concretely. According to the embodiment, the brake ECU  17  judges that the out-flow liquid amount is greater than the predetermined out-flow amount, when the control state of the actuator  16  to all of the front wheels Wf is a pressure increasing state and that the presumed pressure or the measured pressure of the wheel cylinders WCf of the front wheels Wf is less than a predetermined pressure. In other words, in such state, the first control is executed. The wheel cylinder pressure can be presumed by a well-known presumption (calculation) method, such as for example, presumed from the control state of booster mechanism  15  or the actual servo pressure (value of pressure sensor  26   a ), or from the control state of each electromagnetic valve of the actuator  16 . The brake ECU  17  observes and knows the control state of each electromagnetic valve in the actuator  16 . The predetermined pressure is set in advance. If the vehicle is equipped with pressure sensors which measure the respective wheel cylinder pressures, such measured pressure can be used for the judgement. It is noted that in the judgement, if the control state of the actuator  16  for at least one of the front wheels Wf is in the pressure increasing state and that the presumed pressure or the measured pressure of the wheel cylinder WCf of corresponding front wheel Wf is less than the predetermined pressure, it is judged that the out-flow liquid amount is greater than the predetermined out-flow amount. 
     Regarding to the wheel cylinder WC, the relation between the flow-rate and the pressure is confirmed already in advance and generally, the smaller the pressure, the larger the flow-rate necessary for raising the pressure becomes. In other words, the smaller the wheel cylinder pressure, the larger the in-flow liquid amount to the wheel cylinder WC easily becomes. Accordingly, it is judged that the out-flow liquid amount is greater than the out-flow amount when the presumed pressure of the wheel cylinder WC (may be also referred to as “presumed wheel cylinder pressure”) is less than the predetermined pressure. 
     Further, the brake ECU  17  judges also that the out-flow liquid amount is greater than the predetermined out-flow amount when the presumed in-flow liquid amount per unit time to the wheel cylinder WC to which the ABS control is executed is greater than a predetermined value. In other words, in this case, also, the first control is executed. The in-flow liquid amount (cc/s) to the wheel cylinder WC can be presumed by a well-known presumption (calculation) method, such as for example, presumed from the control state of each electromagnetic valve of the actuator  16 , the control state of the booster mechanism  15  or the measured value of the servo pressure and a presumed wheel cylinder pressure (or the measured wheel cylinder pressure). The brake ECU  17  makes the judgment by comparing the calculated presumed in-flow liquid amount with the predetermined value set in advance. 
     Further, the brake ECU  17  judges that the in-flow liquid amount is greater than the predetermined in-flow amount when the ejected amount (cc/s) of the brake liquid per unit time ejected by the pumps  46  and  56  is greater than a predetermined elected amount. In other words, in this case the second control is executed. Since the brake ECU  17  controls the driving operation of the pumps  46  and  56 , the brake ECU  17  can confirm the ejected amount of the brake liquid per unit time ejected by the pumps  46  and  56 . 
     Hereinafter, the first and the second controls will be explained by raising a concrete control example. First, a case that neither the first control nor the second control is executed will be explained. As shown at the upper portion in  FIG. 2 , when the brake operation starts, a regeneration braking operation is initiated by the regeneration braking device B. In this case, almost all of the required barking force (values corresponding to the brake operation) are covered by the regeneration barking force and accordingly, the use of hydraulic pressure braking force generated by the wheel cylinder pressure is substantially zero. The brake ECU  17  controls the master pressure by the booster mechanism  15  so that the difference (insufficient braking force) between the required braking force and the regeneration braking force is appropriated by the hydraulic pressure braking force. In this example, the master pressure becomes substantially zero (atmospheric pressure). However, the master pressure is not necessarily zero. Under this situation, the braking force applied to the front wheel Wf is the sum of the regeneration braking force and the master pressure (=wheel cylinder pressure). Further, the braking force applied to the rear wheel Wr corresponds to the hydraulic pressure braking force (=wheel cylinder pressure). Under this situation, the vehicle becomes a front loaded state (state where the front side of the vehicle is submerged). 
     Then, as shown at middle portion in  FIG. 2 , when the ABS control is executed only to the front wheel Wf, the regeneration braking force is released and the master pressure (for example, a hydraulic pressure which exerts the hydraulic pressure braking force equal to or the same with the regeneration braking force) which has been adjusted by the booster mechanism  15  is supplied to the wheel cylinder WC. Under such state, the pressure decreasing control is performed to the wheel cylinder WCf of the front wheel Wf and the master pressure is not supplied to the wheel cylinder WCf to thereby eject the fluid in the wheel cylinders WCf to the master chamber R 1  and R 2  side by the operation of the pumps  46  and  56 . The master pressure under this state is the sum of the hydraulic pressure adjusted by the booster mechanism  15  and exerting the hydraulic pressure braking force which is equal to or the same with the regeneration braking force and an increased pressure worth increased based on the ejected amount of brake liquid ejected by the pumps  46  and  56 . The state of the wheel cylinder pressure at the front wheel Wf is kept to the pressure decreased state until the slipping of vehicle wheel recovers. On the other hand, the state of the wheel cylinder pressure at the rear wheel Wr becomes the master pressure. In other words, the braking force at the rear wheel Wr suddenly becomes greater than the braking force at the front wheel Wf and the vehicle becomes a rear loaded state (state where the rear side of the vehicle is submerged). 
     Next, as shown at the lower portion in  FIG. 2 , when only the front wheel Wf is under ABS control, the brake ECU  17  executes a pressure increasing control to the wheel cylinder WCf to apply braking force in response to the road surface condition (in response to the friction coefficient on the road surface). Thus, the brake liquid in the master chambers R 1  and R 2  flows into the wheel cylinder WC of the front wheel Wf to decrease the master pressure corresponding thereto. This pressure decreasing operation of the master pressure makes the braking force at the rear wheel Wr to drop and the vehicle again becomes the front loaded state. As explained, when the ABS control is executed only to a portion of the wheel W (in this case, the front wheel Wf), the vehicle has a tendency of making pitching in a front/rear direction and an improvement with respect to the stability of the vehicle behavior is still needed. 
     It is noted here that hereinafter, the explanation of the case where the first and the second controls are executed will be made. As shown in  FIG. 3 , when the ABS control is performed only to the front wheel Wf and the regeneration braking force operation stops, in order to output the regeneration braking force worth of the hydraulic pressure braking force, the master pressure is increased by the booster mechanism  15  and at the same time the pressure decreasing control to the front wheel Wf is executed. At this time, the brake ECU  17  monitors the ejected amount of the pumps  46 ,  56 , and executes the second control when the ejected amount per unit time is greater than a predetermined ejected amount. 
     As shown with the dotted line A 1  in  FIG. 3 , the second control, in the case of pressure increasing control of the master pressure, is a control which changes the control amount of the booster mechanism  15  such that the increase amount of the master pressure per unit time (inclination of pressure increasing) is changed in a reducing direction. In this embodiment the control amount corresponds to the in- and out-flow amount of the brake liquid with respect to the servo chamber R 5 . 
     The control of the master pressure (servo pressure) by the booster mechanism  15  is performed by the combination of the feed-back control and the feed-forward control and for example, performed by PID control (Proportional Integral Derivative Controller). The flow-rate Q of the fluid flowing into the servo chamber R 5  increases as the difference ΔP between the target servo pressure (target master pressure) and the actual servo pressure (value of the pressure sensor  26   a ) becomes large. The flow-rate Q is for example, set by the value (Q=K p ×ΔP+K D ×Z 1 +K I ×Z 2 ). In this formula, the values of K p , K D  and K I  are the set coefficient values, Z 1  indicates the servo pressure change amount (differential value) and Z 2  indicates the servo pressure integrated value. In the second control where the master pressure is under pressure increasing control, for example, the value K p  is set to be smaller than the set value (initial value). In other words, the brake ECU  17  switches over the feedback gain to a smaller value (set value of the second control) which is smaller than a value at normal control operation. Therefore, the flow-rate Q becomes small compared to the case that second control is not being performed and the increasing amount of the master pressure per unit time becomes smaller. 
     Further, as shown with the dotted line A 2  in  FIG. 3 , the second control, in the case of pressure decreasing control of the master pressure, is a control which changes the control amount of the booster mechanism  15  such that the decrease amount of the master pressure per unit time (inclination of pressure decreasing) is changed in an increasing direction. For example, the brake ECU  17  switches over the set coefficient (for example, feedback gain) (for example, to a larger value) by performing the second control and controls the booster mechanism  15  such that the decreasing amount of the master pressure per unit time becomes large, larger than a value at normal control operation. Therefore, the decreasing amount of the master pressure per unit time becomes larger than a case where the second control is not being performed. 
     Consequently, under the ABS control at the front wheel Wf, control is changed from the pressure decreasing control to the pressure increasing control, and the first and the second pressure increasing control valves  41  and  42  are in the open state. Under this situation, when the presumed in-flow liquid amount (cc/s) per unit time flowing into the wheel cylinder WCf is larger than a predetermined value or the presumed wheel cylinder pressure of the front wheel Wf is less than a predetermined pressure, the brake ECU  17  executes the first control. The presumed in-flow liquid amount may be a passing flow-rate of fluid per unit time passing through the first and the second pressure increasing control valves  41  and  42 . 
     As shown with the dotted line B 1  in  FIG. 3 , the first control in the case of pressure decreasing control of the master pressure is a control which changes the control amount of the booster mechanism  15  such that the decrease amount of the master pressure per unit time (inclination of pressure decreasing) is changed in a decreasing direction. As similar to the case of the second control, the brake ECU  17  switches over the set coefficient (for example, feedback gain) (for example, to a smaller value) and controls the booster mechanism  15  such that the decreasing amount of the master pressure per unit time becomes small, smaller than a value at normal control operation. Therefore, the decreasing amount of the master pressure per unit time becomes smaller than a case where the first control is not being performed. 
     Further, as shown with the dotted line B 2  in  FIG. 3 , the first control, in the case of pressure increasing control of the master pressure, is a control which changes the control amount of the booster mechanism  15  such that the increase amount of the master pressure per unit time (inclination of pressure increasing) is changed in an increasing direction. As similar to the case of the second control, the brake ECU  17  switches over the set coefficient (for example, feedback gain) (for example, to a larger value) and controls the booster mechanism  15  such that the increasing amount of the master pressure per unit time becomes large, larger than a value at normal control operation. Therefore, the increasing amount of the master pressure per unit time becomes larger than a case where the first control is not being performed. 
     In other words, the first control can be said that an instruction to strengthen the pressure increasing operation (instruct the valve to further open) is sent to the pressure increasing valve  15   b   7  during the master pressure increasing operation and that an instruction to weaken the pressure decreasing operation (instruct the valve to further close) is sent to the pressure decreasing valve  15   b   6  during the master pressure decreasing operation. Further, the second control can be said that an instruction to weaken the pressure increasing operation (instruct the valve to further close) is sent to the pressure increasing valve  15   b   7  during the master pressure increasing operation and that an instruction to strengthen the pressure decreasing operation (instruct the valve to further open) is sent to the pressure decreasing valve  15   b   6  during the master pressure decreasing operation. It is noted that the first control and the second control explained above as an example, do not change the target master pressure (target servo pressure). 
     An example of the flow of control will be explained hereinafter with reference to  FIG. 4 . The brake ECU  17  judges whether or not the control state is the state that ABS control is being executed only to one or some of the wheels W (S 101 ). If the control state is judged to be the state that the ABS control is being executed only to the one or some of the wheels W (S 101 ; Yes), the brake ECU  17  judges whether or not the presumed in-flow liquid amount of fluid per unit time of the wheel cylinder WC to which the ABS control is executed is larger than a predetermined value (S 102 ). If the presumed in-flow liquid amount is judged to be larger than the predetermined value (S 102 ; Yes), the brake ECU  17  executes the first control according to the control state of the master pressure (S 103 ). 
     On the other hand, if the presumed in-flow liquid amount is judged to be equal to or less than the predetermined value (S 102 ; No), the brake ECU  17  judges whether or not the ejecting amount of fluid per unit time by the pumps  46 ,  56  is greater than a predetermined ejecting amount (S 104 ). If the ejecting amount is greater than the predetermined ejecting amount, (S 104 ; Yes), the brake ECU  17  executes the second control according to the control state of the master pressure (S 105 ). 
     If all of the wheels “W” are under execution of ABS control, or none of the wheels “W” are under execution of ABS control (S 101 ; No), or the ejecting amount of fluid is equal to or less than the predetermined ejecting amount (S 104 ; No), the first control and the second control are not executed and execution of normal control is kept continuing. The brake ECU  17  can executes the above control flow per every predetermined time. It is noted that the first control and the second control are stopped, for example, when the execution condition is cancelled and the set coefficient returns to the normal value.
         (Effect)       

     According to the embodiment, by suppressing the pressure increase of the master pressure or by enhancing the pressure decrease of the master pressure by the execution of the second control, the raise of master pressure caused by the pumping back phenomenon during the ABS control can be suppressed. Accordingly, a sudden raise of braking force at the non-ABS controlled rear wheel Wr can be avoided to suppress an occurrence of transitional braking force imbalance (for example, transition of vehicle state from front loaded to rear loaded state). In other words, according to the second control, the stability of vehicle posture can be improved. Further, according to the embodiment, by suppressing the pressure decrease of the master pressure or by enhancing the pressure increase of the master pressure by the execution of the first control, the drop of master pressure caused by the increase of flow-rate to the wheel cylinders WC to which the ABS control is executed can be suppressed. Accordingly, a drop of braking force at the rear wheel Wr can be avoided to suppress an occurrence of transitional braking force imbalance (for example, transition of vehicle state from rear loaded to front loaded state). In other words, the stability of vehicle posture can be improved also by the first control. As explained, according to the embodiment, the increase or decrease of the master pressure caused by the ABS control to one or some of the wheels can be suppressed and this can eventually contribute to the improvements in vehicle stability during braking operation. 
     Further, according to the embodiment, the timing of execution of the first control is judged based on the presumed pressure of the wheel cylinder pressure or the presumed in-flow liquid amount and the timing of execution of the second control is judged based on the ejecting amount of fluid by the pumps  46  and  56 . Thus, the first and the second controls are executed at an appropriate timing in response to the current state. 
     Further, since the vehicle according to the embodiment is a hybrid vehicle which generates a regeneration braking force to the front wheel Wf, initial behavior of vehicle by the braking operation tends to make the vehicle to be in a front loaded state and further, since the initial ABS control is executed only to the front wheel Wf, the vehicle behavior shown in  FIG. 2  tends to be generated. Further, according to this vehicle, it is necessary to increase the master pressure greatly after the regeneration braking operation is released and upon this situation, the behavior shown in  FIG. 2  tends to be generated. Accordingly, the first control and the second control according to the embodiment are particularly very effective to the vehicle equipped with a regeneration braking device. In other words, the embodiment of the invention is further effective to the vehicle which is capable of generating a regenerative braking force and further more effective to the vehicle which is equipped with a regeneration braking device which applies the regeneration braking force to the front wheel Wf. It is noted however, even a vehicle with no such regeneration braking device can suppress the generation of imbalance of transitional braking force caused by the pressure increase or decrease of master pressure upon ABS controlling to one or some of the wheels W, as long as a master chamber common to (when a master chamber is divided into a plurality of chambers, if such chambers are mechanically inter-connected, such chambers may be said to be one master chamber) a plurality of wheel cylinders WC according to the embodiment. Therefore, the stability of vehicle behavior can be improved for such vehicle by performing the first and the second controls.
         (Others)       

     The present invention is not limited to the embodiment explained above, but may include a structure wherein the booster mechanism  15  does not have the regulator  15   a . The booster mechanism  15  is, for example, configured to have the pressure increasing valve connected to the high pressure source and the pressure decreasing valve connected to the low pressure source for controlling the fluid in the servo chamber R 5 . Further, the booster mechanism  15  may be a booster mechanism which drives the first master piston  12   c  by control and may be configured by a motor and a ball screw, etc., which drives the first master piston  12   c  by being driven by the motor. In such configuration, the control amount of the motor (ball screw displacement amount) corresponds to the in and out-flow of fluid (controlled flow-rate) into or out of the servo chamber R 5 . 
     Further, the first control may be set such that the target servo pressure (target master pressure) is raised temporarily and the second control may be set such that the target servo pressure (target master pressure) is dropped temporarily. Still further, the brake ECU  17  may be set such that the brake ECU  17  may execute only one of the first and the second controls. This invention is applicable to a vehicle which is not equipped with a regeneration braking device. 
     Further, the “out-flow liquid amount of brake liquid per unit time flowing out of the master chambers R 1  and R 2 ” may be set to the integrated amount (integrated value) of fluid flowing out of the master chambers R 1  and R 2  after the ABS control started. The judgment may be made based on such integrated amount of fluid. The predetermined out-flow amount may be set based on the integrated value. Similarly, the “in-flow liquid amount of brake liquid per unit time flowing into the master chambers R 1  and R 2 ” may be set to the integrated amount (integrated value) of fluid flowing into the master chambers R 1  and R 2  after the ABS control started. The judgment may be made based on such integrated amount of fluid. The predetermined in-flow amount may be set based on the integrated value. It may be said that the out-flow liquid amount per unit time and in-flow liquid amount per unit time may, as a meaning, include conceptually the integrated amount. 
     Further, the judgement of the second control execution may be based on the condition of “during pressure decreasing control in ABS control and when the ejection amount is greater than a predetermined ejection amount”. For example, when the pumps are always operated with a constant rotation speed during the ABS control, the ejecting amount of the pumps  46  and  56  become constant when the fluid supply source exists and accordingly, the ejecting amount may be presumed (judged) by judging whether the pressure decreasing control valves  44 ,  45 ,  54 ,  55  are closed or not (whether or not the control is under pressure decreasing control). In other words, the magnitude of the ejecting amount can be judged whether or not the wheel cylinder WC is under a fluid supply source state. 
     REFERENCE SIGNS LIST 
       11 ; brake pedal (brake operating member),  12 : master cylinder,  12   c : first master piston,  12   d : second master piston,  15 ; booster mechanism (driving portion),  16 ; actuator,  46 ,  56 : pump,  17 ; brake ECU (control portion), “A”; vehicle braking device, R 1 : first master chamber, R 2 : second master chamber, R 5 ; servo chamber, W: vehicle wheel, WC; wheel cylinder.