Patent Publication Number: US-2020290581-A1

Title: Breaking Device and Breaking System

Description:
TECHNICAL FIELD 
     The present invention relates to a braking device. 
     BACKGROUND ART 
     Hitherto, there has been known a braking device, which includes a pump and is configured to supply a brake fluid to wheel cylinders. For example, in Patent Literature 1, a piston pump applied to a braking device is disclosed. 
     CITATION LIST 
     Patent Literature 
     PTL 1: DE 19948445 A1 
     SUMMARY OF INVENTION 
     Technical Problem 
     Improvement in boost responsiveness of the wheel cylinders is desired. The present invention has an object to provide a braking device capable of improving the boost responsiveness. 
     Solution to Problem 
     According to one embodiment of the present invention, there is provided a braking device including a second chamber from which a brake fluid is discharged by a movement of a piston caused by inflow of the brake fluid flowed out from a master cylinder to a first chamber through a brake operation by a driver, and a pump configured to discharge the brake fluid into an oil passage for supplying the brake fluid flowed out from the second chamber to a wheel cylinder. 
     Thus, the boost responsiveness of the wheel cylinders can be increased. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic configuration diagram for illustrating a braking system according to a first embodiment. 
         FIG. 2  is a perspective view for illustrating a part of the braking system according to the first embodiment. 
         FIG. 3  is a sectional view for illustrating a first unit in the first embodiment. 
         FIG. 4  is a front transparent view for illustrating a housing of a second unit in the first embodiment. 
         FIG. 5  is a rear transparent view for illustrating the housing of the second unit in the first embodiment. 
         FIG. 6  is a top transparent view for illustrating the housing of the second unit in the first embodiment. 
         FIG. 7  is a bottom transparent view for illustrating the housing of the second unit in the first embodiment. 
         FIG. 8  is a right side transparent view for illustrating the housing of the second unit in the first embodiment. 
         FIG. 9  is a left side transparent view for illustrating the housing of the second unit in the first embodiment. 
         FIG. 10  is a front view for illustrating the second unit in the first embodiment. 
         FIG. 11  is a rear view for illustrating the second unit in the first embodiment. 
         FIG. 12  is a right side view for illustrating the second unit in the first embodiment. 
         FIG. 13  is a left side view for illustrating the second unit in the first embodiment. 
         FIG. 14  is a top view for illustrating the second unit in the first embodiment. 
         FIG. 15  is a sectional view as viewed in a direction indicated by the line XV-XV of  FIG. 14 , 
         FIG. 16  is a rear view for illustrating the second unit in the first embodiment in a state in which a case lid part of an ECU is removed. 
         FIG. 17  is a graph for showing a relationship between a rotation angle and a load torque in a first example in which the number of pump parts is two. 
         FIG. 18  is a graph for showing the relationship between the rotation angle and the load torque in a second example in which the number of the pump parts is three. 
         FIG. 19  is a graph for showing the relationship between the rotation angle and the load torque in a third example in which the number of the pump parts is four. 
         FIG. 20  is a graph for showing the relationship between the rotation angle and the load torque in a fourth example in which the number of the pump parts is five. 
         FIG. 21  is a graph for showing the relationship between the rotation angle and the load torque in a fifth example in which the number of the pump parts is six. 
         FIG. 22  is a right side view for illustrating the second unit of the first embodiment with transparency in the housing. 
         FIG. 23  is a front transparent view for illustrating the housing of the second unit in a second embodiment. 
         FIG. 24  is a transparent perspective view for illustrating the housing of the second unit in the second embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Now, embodiments of the present invention are described based on the drawings. 
     First Embodiment 
     First, description is given of a configuration.  FIG. 1  is a diagram for illustrating a schematic configuration of a braking system  1  according to the first embodiment together with a hydraulic circuit.  FIG. 2  is a perspective view for illustrating a part of the braking system  1 . The braking system  1  is applied to an electrically driven vehicle. The electrically driven vehicle refers to, for example, a hybrid vehicle including an electric motor (generator) in addition to an internal combustion engine (engine) as a motor for driving wheels, or an electric automobile including only an electric motor (generator) as a motor for driving wheels. In the electrically driven vehicle, regenerative braking, that is, breaking of the vehicle by regenerating electric energy from kinetic energy of the vehicle can be performed with use of a regenerative braking device including a motor (generator). The braking system  1  is a hydraulic pressure braking device configured to apply friction braking forces through hydraulic pressures to wheels FL to RR of the vehicle. A brake operation unit is provided for each of the wheels FL to RR. The brake operation unit is a hydraulic pressure generation part including a wheel cylinder W/C. The brake operation unit is of, for example, a disc type, and includes a caliper (hydraulic brake caliper). The caliper includes a brake disc and brake pads. The brake disc is a brake rotor rotating integrally with a tire. The brake pads are arranged so as to have predetermined clearances to the brake disc, and are moved by the hydraulic pressures of the wheel cylinder W/C, to thereby come into contact, with the brake disc. As a result, a friction braking force is generated. The braking system  1  includes two systems (primary P system and secondary S system) of brake pipes. The brake pipe type is, for example, an X-split pipe type. Other pipe types such as a front/rear-split pipe may be employed. Hereinafter, when a member correspondingly provided to the P system and a member correspondingly provided to the S system are distinguished from one other, suffixes P and S are added to respective reference symbols. The braking system  1  is configured to supply the brake fluid serving as working fluid (working oil) to each of the brake operation units through the brake pipes, to thereby generate hydraulic pressures (brake hydraulic pressures) in the wheel cylinders W/C. As a result, a hydraulic pressure braking force is applied to each of the wheels FL to RR. 
     The braking system  1  includes a first unit  1 A and a second unit  1 B. The first unit  1 A and the second unit  1 B are provided in a motor room isolated from a cabin of the vehicle, and are connected to each other by a plurality of pipes. The plurality of pipes include master cylinder pipes  10 M (primary pipe  10 MP and secondary pipe  10 MS), wheel cylinder pipes  10 W, a back pressure pipe  10 X, and a suction pipe  10 R. Each of the pipes  10 M,  10 W, and  10 X other than the suction pipe  10 R is a brake pipe made of metal (metal pipe), specifically, for example, a double-wound steel pipe. Each of the pipes  10 M,  10 W, and  10 X has straight portions and bent portions, and is arranged between ports while the direction is changed at the bent portions. Both ends of each of the pipes  10 M,  10 W, and  10 X include flared male pipe joints. The suction pipe  10 R is a brake hose (hose pipe) made of a material such as rubber so as to be flexible. Ends of the suction pipe  10 R are connected to a port  873  and the like by nipples  10 R 1  and  10 R 2 . The nipples  10 R 1  and  10 R 2  are resin connection members including pipe portions. 
     A brake pedal  100  is a brake operation member configured to receive an input of a brake operation by a driver. A pushrod  101  is rotatably connected to the brake pedal  100 . The first unit  1 A is a brake operation unit mechanically connected to the brake pedal  100 , and is a master cylinder unit including a master cylinder  5 . The first unit  1 A includes a reservoir tank  4 , a housing  7 , the master cylinder  5 , a stroke sensor  94 , and a stroke simulator  6 . The reservoir tank  4  is a brake fluid source for reserving the brake fluid, and is a low-pressure part opened to the atmospheric pressure. Supplement ports  40  and a supply port  41  are formed in the reservoir tank  4 . The suction pipe  10 R is connected to the supply port  41 . The housing  7  is a casing for accommodating (build in) the master cylinder  5  and the stroke simulator  6  therein. A cylinder  70  for the master cylinder  5 , a cylinder  71  for the stroke simulator  6 , and a plurality of oil passages (liquid passages) are formed in the housing  7 . The plurality of oil passages include supplement oil passages  72 , supply oil passages  73 , and a positive pressure oil passage  74 . A plurality of ports are formed in the housing  7 , and those ports are opened in outer surfaces of the housing  7 . The plurality of ports include supplement ports  75 P and  75 S, supply ports  76 , and a back pressure port  77 . The supplement ports  75 P and  75 S are connected to supplement ports  40 P and  40 S of the reservoir tank  4 , respectively. The master cylinder pipes  10 M are connected to the supply ports  76 , and the back pressure pipe  10 X is connected to the back pressure port  77 . One end of the supplement oil passage  72  is connected to the supplement port  75 , and the other end is connected to the cylinder  70 . 
     The master cylinder  5  is a first hydraulic pressure source capable of supplying an operation hydraulic pressure to the wheel cylinders W/C. The master cylinder  5  is connected to the brake pedal  100  via the pushrod  101 , and is operated in accordance with an operation on the brake pedal  100  by the driver. The master cylinder  5  includes a piston  51  which is moved in an axial direction in accordance with the operation on the brake pedal  100 . The piston  51  is accommodated in the cylinder  70 , and defines hydraulic pressure chambers  50 . The master cylinder  5  is of a tandem type, and includes, as pistons  51 , a primary piston  51 P connected to the pushrod  101  and a secondary piston  51 S of a free piston type in series. A primary chamber  50 P is defined by the pistons  51 P and  51 S, and a secondary chamber  50 S is defined by the secondary piston  51 S. One end of the supply oil passage  73  is connected to the hydraulic pressure chamber  50 , and the other end is connected to the supply port  76 . Each of the hydraulic pressure chambers  50 P and  50 S is supplemented with the brake fluid from the reservoir tank  4  to generate a hydraulic pressure (master cylinder pressure) through the movement of the piston  51 . The stroke sensor  94  is configured to detect a stroke (pedal stroke) of the primary piston  51 P. A magnet for detection is provided in the primary piston  51 P, and a sensor main body is mounted to an outer surface of the housing  7  of the first unit  1 A. 
     The stroke simulator  6  is operated in accordance with the brake operation by the driver, and is configured to apply a reaction force and a stroke to the brake pedal  100 . The stroke simulator  6  includes a piston  61 , a positive pressure chamber  601  and a back pressure chamber  602  defined by the piston  61 , and an elastic body (spring  64  or the like) configured to bias the piston  61  in a direction in which the volume of the positive pressure chamber  601  decreases. One end of the positive pressure oil passage  74  is connected to a supply oil passage  73 S on the secondary side, and the other end is connected to the positive pressure chamber  601 . The pedal stroke is generated by inflow of the brake fluid from the master cylinder  5  (secondary chamber  50 S) to the positive pressure chamber  601  in accordance with the brake operation by the driver, and a reaction force against a brake operation by the driver is generated by the biasing force of the elastic body. The first unit  1 A does not include an engine negative pressure booster configured to boost the brake operation force through use of an intake negative pressure generated in the engine of the vehicle. 
     The second unit  1 B is a hydraulic pressure control unit provided between the first unit  1 A and the brake operation units. The second unit  1 B is connected to the primary chamber  50 P by the primary pipe  10 MP (first pipe), is connected to the secondary chamber  50 S by the secondary pipe  10 MS (first pipe), is connected to the wheel cylinders W/C by the wheel cylinder pipes  10 W (second pipes), and is connected to the back pressure chamber  602  by the back pressure pipe  10 X (third pipe). Moreover, the second unit  1 B is connected to the reservoir tank  4  by the suction pipe  10 R. The second unit  1 B includes a housing  8 , a motor  20 , a pump  3 , a plurality of electromagnetic valves  21 , a plurality of hydraulic pressure sensors  91 , and an electronic control unit  90  (control unit, hereinafter referred to as “ECU”). The housing  8  is a casing for accommodating (build in) the pump  3 , valve bodies of the electromagnetic valves  21 , and the like therein. Circuits (brake hydraulic pressure circuits) of the two systems (P system and S system), through which the brake fluid circulates, are formed of a plurality of oil passages in the housing  8 . The plurality of oil passages include supply oil passages  11 , a suction oil passage  12 , discharge oil passages  13 , a pressure regulating oil passage  14 , pressure reducing oil passages  15 , a back pressure oil passage  16 , a first simulator oil passage  17 , and a second simulator oil passage  18 . Moreover, a reservoir (internal reservoir)  120 , which is a liquid reservoir, and a damper  130  are formed in the housing  8 . A plurality of ports are formed in the housing  8 , and those ports are opened in outer surfaces of the housing  8 . The plurality of ports include master cylinder ports  871  (primary ports  871 P and secondary ports  871 S), a suction port  873 , a back pressure port  874 , and wheel cylinder ports  872 . The primary pipe  10 MP, the secondary pipe  10 MS, the suction pipe  10 R, the back pressure pipe  10 X, and the wheel cylinder pipes  10 W are mounted and connected to the primary port  871 P, the secondary port  871 S, the suction port  873 , the back pressure port  874 , and the wheel cylinder ports  872 , respectively. 
     The motor  20  is an electric motor of a rotation type, and includes a rotation shaft configured to drive the pump  3 . The motor  20  may be a brushless motor or a brush motor. The motor  20  includes a resolver configured to detect a rotation angle of the rotation shaft. The resolver functions as a number-of-revolution sensor configured to detect the number of revolutions of the motor  20 . The pump  3  is a hydraulic pressure source capable of supplying an operation hydraulic pressure to the wheel cylinders W/C, and includes five pump parts  3 A to  3 E driven by the single motor  20 . The pump  3  is used for the S system and the P system in common. Each of the electromagnetic valves  21  and the like is an actuator configured to operate in accordance with a control signal, and includes a solenoid and a valve body. The valve body is configured to perform a stroke in accordance with a current supply to the solenoid to switch opening and closing of an oil passage (open/close the oil passage). Each of the electromagnetic valves  21  and the like controls the communication state of the circuit and adjusts the circulation state of the brake fluid to generate a control hydraulic pressure. The plurality of electromagnetic valves  21  and the like include shutoff valves  21 , pressure boosting valves (hereinafter referred to as “SOL/V IN”)  22 , communication valves  23 , a pressure regulating valve  24 , pressure reducing valves (hereinafter referred to as “SOL/V OUT”)  25 , a stroke simulator-in valve (hereinafter referred to as “SS/V IN”)  27 , and a stroke simulator-out valve (hereinafter referred to as “SS/V OUT”)  28 . Each of the shutoff valve  21 , the SOL/V IN  22 , and the regulating valve  24  is a normally-open valve which is opened in a non-current supply state. Each of the communication valve  23 , the pressure reducing valve  25 , the SS/V IN  27 , and the SS/V OUT  28  is a normally-closed valve, which is closed in the non-current supply state. Each of the shutoff valve  21 , the SOL/V IN  22 , and the pressure regulating valve  24  is a proportional control valve which has an opening degree adjusted in accordance with the current supplied to the solenoid. Each of the communication valve  23 , the pressure reducing valve  25 , the SS/V IN  27 , and the SS/V OUT  28  is an ON/OFF valve which is subjected to binary switching control between an opening state and a closing state. A proportional control valve may be used for each of those valves. Each of the hydraulic pressure sensor  91  and the like is configured to detect a discharge pressure of the pump  3  or a master cylinder pressure. The plurality of hydraulic pressure sensors include a master cylinder pressure sensor  91 , a discharge pressure sensor  93 , and wheel cylinder pressure sensors  92  (primary pressure sensor  92 P and secondary pressure sensor  92 S). 
     Now, based on  FIG. 1 , description is given of the brake hydraulic pressure circuit of the second unit  1 B. For members corresponding to the respective wheels FL to RR, suffixes of “a” to “d” are added to respective reference symbols for proper distinction. One end side of a supply oil passage  11 P is connected to the primary port  871 P. The other end side of the supply oil passage  11 P is branched into an oil passage  11   a  for the front left wheel and an oil passage  11   d  for the rear right wheel. Each of the oil passages  11   a  and  11   d  is connected to the corresponding wheel cylinder port  872 . One end side of a supply oil passage  11 S is connected to the secondary port  871 S. The other end side of the supply oil passage  11 S is branched into an oil passage  11   b  for the front right wheel and an oil passage  11   c  for the rear left wheel. Each of the oil passages  11   b  and  11   c  is connected to the corresponding wheel cylinder port  872 . The shutoff valve  21  is provided on the one end side of each of the supply oil passages  11 . The SOL/V IN  22  is provided on the other end side of each of the oil passages  11 . A bypass oil passage  110  configured to bypass the SOL/V IN  22  is provided in parallel with each of the oil passages  11 . A check valve  220  is provided in the bypass oil passage  110 . The check valve  220  permits only a flow of the brake fluid from the wheel cylinder port  872  side to the master cylinder port  871  side. 
     The suction oil passage  12  connects the reservoir  120  and suction ports  823  of the pump  3  to each other. One end side of the discharge oil passage  13  is connected to discharge ports  821  of the pump  3 . The other end side of the discharge oil passage  13  is branched into the oil passage  13 P for the P system and the oil passage  13 S for the S system. Each of the oil passages  13 P and  13 S is connected to a portion between the shutoff valve  21  and the SOL/V IN  22  in the supply oil passage  11 . A damper  130  is provided on the one end side of the discharge oil passage  13 . The communication valve  23  is provided in each of the oil passages  13 P and  13 S on the other end side. The respective oil passages  13 P and  13 S function as communication passages for connecting the supply oil passage  11 P in the P system and the supply oil passage  11 S in the S system to each other. The pump  3  is connected to the respective wheel cylinder ports  872  by the communication passages (discharge oil passages  13 P and  13 S) and the supply oil passages  11 P and  11 S. The pressure regulating oil passage  14  connects an intermediate portion of the discharge oil passages  13  between the damper  130  and the communication valves  23 , and the reservoir  120  to each other. The pressure regulating valve  24  serving as a first pressure reducing valve is provided in the pressure regulating passage  14 . The pressure reducing oil passage  15  connects an intermediate portion between the SOL/V IN  22  in each of the oil passages  11   a  to  11   d  of the supply oil passage  11  and the wheel cylinder port  872 , and the reservoir  120  to each other. The SOL/V OUT  25  serving as a second pressure reducing valve is provided in the pressure reducing oil passage  15 . 
     One end side of the back pressure oil passage  16  is connected to the back pressure port  874 . The other end side of the back pressure oil passage  16  is branched into a first simulator oil passage  17  and a second simulator oil passage  18 . The first simulator oil passage  17  is connected a portion between the shutoff valve  21 S and the SOL/V IN  22   b  and  22   c  in the supply oil passage  11 S. The SS/V IN  27  is provided in the first simulator oil passage  17 . A bypass oil passage  170  configured to bypass the SS/V IN  27  is provided in parallel with the first simulator oil passages  17 . A check valve  270  is provided in the bypass oil passage  170 . The check valve  270  permits only a flow of the brake fluid from the back pressure oil passage  16  side to the supply oil passage  11 S side. The second simulator oil passage  18  is connected to the reservoir  120 . The SS/V OUT  28  is provided in the second simulator oil passage  18 . A bypass oil passage  180  configured to bypass the SS/V OUT  28  is provided in parallel with the second simulator oil passages  18 . A check valve  280  is provided in the bypass oil passage  180 . The check valve  280  permits only a flow of the brake fluid from the reservoir  120  side to the back pressure oil passage  16  side. 
     A hydraulic pressure sensor  91  is provided at an intermediate position between the shutoff valve  21 S in the supply oil passage  11 S and the secondary port  871 S. The hydraulic pressure sensor  91  is configured to detect a hydraulic pressure at this position (hydraulic pressure in the positive pressure chamber  601  of the stroke simulator  6 , or the master cylinder pressure). A hydraulic pressure sensor  92  is provided at an intermediate position between the shutoff valve  21  in the supply oil passage  11  and the SOL/V INs  22 . The hydraulic pressure sensor  92  is configured to detect a hydraulic pressure at this point (corresponding to the wheel cylinder hydraulic pressure). A hydraulic pressure sensor  93  is provided at an intermediate point between the damper  130  in the discharge oil passage  13  and the communication valves  23 . The hydraulic pressure sensor  93  is configured to detect a hydraulic pressure at this point (pump discharge pressure). 
     Next, detailed description is given of the first unit  1 A.  FIG. 3  is a sectional view for illustrating the first unit  1 A. Hereinafter, for convenience of description, a three-dimensional Cartesian coordinate system including an X axis, a Y axis, and a Z axis is given. In a state in which the first unit  1 A is mounted to the vehicle, a Z-axis direction is the vertical direction, and a positive side in the Z-axis direction is a top side in the vertical direction. An X-axis direction is a front/rear direction of the vehicle, and a positive side in the X-axis direction is the vehicle front side. A Y-axis direction is a lateral direction of the vehicle. The pushrod  101  extends from the end on a negative side in the X-axis direction, which is connected to the brake pedal  100 , to the positive side in the X-axis direction. A rectangular plate-like flange part  78  is provided at an end on the negative side in the X-axis direction of the housing  7 . Bolt holes are formed in four corners of the flange part  78 . A bolt B 1  for fixing and mounting the first unit  1 A to a dash panel on a vehicle body side passes through the bolt hole. The reservoir tank  4  is provided on the positive side in the Z-axis direction of the housing  7 . The reservoir tank  4  is within the width of the flange part  78  in the Y-axis direction. The reservoir tank  4  covers a most part (a part excluding the flange part  78  and an end on the positive side in the X-axis direction) of the housing  7  as viewed from the positive side in the Z-axis direction. A supply port  41  is formed on a surface on a positive side in the Y-axis direction at an end on the negative side in the X-axis direction and on a bottom part side (on the negative side in the Z-axis direction) of the reservoir tank  4 . The nipple  10 R 1  is fixedly provided in the supply port  41 , and one end of the suction pipe  10 R is connected to the nipple  10 R 1 . 
     The cylinder  70  for the master cylinder  5  has a bottomed tubular shape extending in the X-axis direction. A positive side in the X-axis direction of the cylinder  70  is closed and a negative side in the X-axis direction of the cylinder  70  is opened. The cylinder  70  includes a small-diameter part  701  on the positive side in the X-axis direction, and a large-diameter part  702  on the negative side in the X-axis direction. The small-diameter part  701  includes two seal grooves  703  and  704  and one port  705  for each of the P and S systems. Each of the seal grooves  703  and  704  and the port  705  has an annular shape extending in a circumferential direction of an axial center of the cylinder  70 . The port  705  is formed between the two seal grooves  703  and  704 . The cylinder  71  for the stroke simulator  6  is arranged on the negative side in the Z-axis direction of the cylinder  70 . The cylinder  71  has a bottomed tubular shape extending in the X-axis direction. A positive side in the X-axis direction of the cylinder  71  is closed and a negative side in the X-axis direction of the cylinder  71  is opened. The cylinder  71  includes a small-diameter part  711  on the positive side in the X-axis direction, and a large-diameter part  712  on the negative side in the X-axis direction. The cylinders  70  and  71  are within the width of the flange part  78  in the Y-axis direction. 
     The supply port  76 S on the secondary side and both the supplement ports  75  are formed on a surface on the positive side in the Z-axis direction of the housing  7 . The supply port  76 S is formed at an end on the positive side in the X-axis direction of the housing  7 . One end of the secondary pipe  10 MS is fixedly provided in the supply port  76 S. The supplement port  75 S on the secondary side is formed on the negative side in the X-axis direction with respect to the supply port  76 S. The supplement port  75 P on the primary side is formed on the negative side in the X-axis direction with respect to the supplement port  75 S. The supply port  76 P on the primary side and the back pressure port  77  are formed on a surface (side surface) on the positive side in the Y-axis direction of the housing  7 . The supply port  76 P is formed at a position partially overlapping in the X-axis direction with the supplement port  75 S on the secondary side, on the positive side in the Z-axis direction on the above-mentioned surface. One end of the primary pipe  10 MP is fixedly provided in the supply port  76 P. Specifically, a pipe joint at the end of the primary pipe  10 MP is fitted to the supply port  76 P, is sandwiched between a hexagon nut and the housing  7 , and is fixed through tightening, and, consequently, the end is connected to the supply port  76 P. Hereinafter, the other end of the primary pipe  10 MP, and both ends of the metal pipes  10 MS,  10 W, and  10 X are connected to the ports in the same manner. 
     The back pressure port  77  is formed on the negative side in the Z-axis direction with respect to the supply port  76 S on the secondary side, and partially overlaps in the X-axis direction with the supplement port  75 P on the primary side. One end of the back pressure pipe  10 X is fixedly provided in the back pressure port  77 . A supplement oil passage  72 P on the primary side extends from the supplement port  75 P on the primary side to the negative side in the Z-axis direction, and is opened in a port  705 P. A supplement oil passage  72 S on the secondary side extends from the supplement port  75 S on the secondary side to the negative side in the Z-axis direction, and is opened in a port  705 S. A supplement oil passage  73 P on the primary side extends from the supplement port  76 P on the primary side to a negative side in the Y-axis direction, and is opened in the small-diameter part  701  of the cylinder  70 . The supply oil passage  73 S on the secondary side extends from the supply port  76 S on the secondary side to the negative side in the Z-axis direction, and is opened in (an end on the positive side in the X-axis direction of) the small-diameter part  701  of the cylinder  70 . The positive pressure oil passage  74  includes a part  741  extending from an end on the positive side in the X-axis direction of the small-diameter part  711  to the negative side in the Z-axis direction, and a part  742  extending from an end on the negative side in the Z-axis direction of the part  741  to the negative side in the X-axis direction, and is connected to an end on the positive side in the X-axis direction of the cylinder  71 . 
     Each of the pistons  51  has a bottomed tubular shape, and is accommodated in the cylinder  70 . The pistons  51 P and  51 S can move in the X-axis direction along an inner peripheral surface of the small-diameter part  701 . The piston  51  includes a first recessed part  511  and a second recessed part  512  having a partition wall  510  as a common bottom part. A hole  513  passes through a peripheral wall of the first recessed part  511 . The first recessed part  511  is formed on the positive side in the X-axis direction, and the second recessed part  512  is formed on the negative side in the X-axis direction. A positive side in the X-axis direction of the pushrod  101  is accommodated in the second recessed part  512 P of the primary piston  51 P. A semispherical round end of the pushrod  101  on the positive side in the X-axis direction abuts against the partition wall  510 P. The pushrod  101  has a flange part  102 . The movement of the pushrod  101  to the negative side in the X-axis direction is restricted by abutment between a stopper member  700  provided in an opening of the cylinder  70  (large-diameter part  702 ) and the flange part  102 . In the small-diameter part  701 , the primary chamber  50 P is defined between the primary piston  51 P (first recessed part  511 P) and the secondary piston  51 S (second recessed part  512 S). The secondary chamber  50 S is defined between the secondary piston  51 S (first recessed part  511 S) and an end on the positive side in the X-axis direction of the small-diameter part  701 . A coil spring  52 P serving as a return spring is provided in the primary chamber  50 P while the coil spring  52 P is compressed between the partition wall  510 P and the partition wall  510 S. A coil spring  52 S serving as a return spring is provided in the secondary chamber  50 S while the coil spring  52 S is compressed between the partition wall  510 S and the end on the positive side in the X-axis direction of the small-diameter part  701 . The supply oil passages  73 P and  73 S are always opened in the chambers  50 P and  50 S, respectively. 
     Seal members  531  and  532  each having a cup shape are provided in the seal grooves  703  and  704 , respectively. A rip part of each of the seal members  531  and  532  is brought into slide contact with an outer peripheral surface of the piston  51 . On the primary side, the seal member  531 P on the negative side in the X-axis direction is configured to suppress a flow of the brake fluid from the positive side in the X-axis direction (port  705 P) to the negative side in the X-axis direction (large-diameter part  702 ). The seal member  532 P on the positive side in the X-axis direction is configured to suppress a flow of the brake fluid to the negative side in the X-axis direction (port  705 P), and permit a flow of the brake fluid to the positive side in the X-axis direction (primary chamber  50 P). On the secondary side, the seal member  531 S on the negative side in the X-axis direction is configured to suppress a flow of the brake fluid from the negative side in the X-axis direction (primary chamber  50 P) to the positive side in the X-axis direction (port  705 S). The seal member  532 S on the positive side in the X-axis direction is configured to suppress a flow of the brake fluid to the negative side in the X-axis direction (port  705 S), and permit a flow of the brake fluid to the positive side in the X-axis direction (secondary chamber  50 S). In an initial state in which both the pistons  51 P and  51 S are maximally displaced to the negative side in the X-axis direction, the hole  513  is positioned (on a side closer to the seal member  532  in the positive side in the X-axis direction) between portions at which both the seal members  531  and  532  (rip parts) and the outer peripheral surface of the piston  51  are in contact with each other. 
     The master cylinder  5  is a hydraulic pressure source that is connected to the wheel cylinders W/C by the primary pipe  10 MP, the secondary pipe  10 MS, the supply oil passages  11 P and  11 S, and the wheel cylinder pipes  10 W, and can increase the wheel cylinder hydraulic pressures. The brake fluid which has flowed out from the master cylinder  5  through the brake operation by the driver flows to the master cylinder pipes  10 M, and is taken into the supply oil passages  11  of the second unit  1 B through the master cylinder ports  871 . The master cylinder  5  can pressurize the wheel cylinders W/C (FL) and W/C (RR) via the oil passage (supply oil passage  11 P) of the P system by the master cylinder pressure generated in the primary chamber  50 P. Simultaneously, the master cylinder  5  can pressurize the wheel cylinders W/C (FR) and W/C (RL) via the oil passage (supply oil passage  11 S) of the S system by the master cylinder pressure generated in the secondary chamber  50 S. 
     The stroke simulator  6  includes a plug member  63 , a piston  61 , a retainer member  62 , a first spring  64 , and a second spring  65 . The plug member  63  closes the opening of the cylinder  71  (large-diameter part  712 ). A first recessed part  631  having a bottomed tubular shape and a second recessed part  632  having a bottomed annular shape are provided on the positive side in the X-axis direction of the plug member  63 . A damper  66  having a cylindrical shape is provided in the first recessed part  631 . The damper  66  is an elastic member made of, for example, rubber. The piston  61  has a bottomed tubular shape having a recessed part, and is accommodated in the cylinder  71 . An opening side of the recessed part is on the positive side in the X-axis direction. A seal groove  610  is formed in an outer peripheral surface of the piston  61 . The piston  61  can move in the X-axis direction along an inner peripheral surface of the small-diameter part  711 . An inside of the cylinder  71  is partitioned and separated into two chambers by the piston  61 . A positive pressure chamber  601  (main chamber) as a first chamber is defined between the positive side in the X-axis direction (recessed part) of the piston  61  and the small-diameter part  711 . A back pressure chamber  602  (sub chamber) as a second chamber is defined between the negative side in the X-axis direction (bottom part) of the piston  61  and the large-diameter part  712 . A seal member (O ring)  67  is provided in the seal groove  610 . The seal member  67  is brought into slide contact with the inner peripheral surface of the small-diameter part  711 . The positive pressure chamber  601  and the back pressure chamber  602  are separated from each other in a liquid tight manner by the seal member  67 . 
     The retainer member  62  has a bottomed tubular shape including a recessed part  620 , and includes a flange part  621  on an opening side of the recessed part  620 . The retainer member  62 , the first spring  64 , and the second spring  65  are accommodated in the back pressure chamber  602 . The first spring  64  is a coil spring having a large diameter, and is an elastic member configured to always bias the piston  61  to the positive pressure chamber  601  (direction of decreasing the volume of the positive pressure chamber  601 , and increasing the volume of the back pressure chamber  602 ). One end of the first spring  64  is held on the first recessed part  631  of the plug member  63 . The first spring  64  is provided in a compressed state between the plug member  63  and the retainer member  62  (flange part  621 ). The retainer member  62  is configured to hold the first spring  64 . The second spring  65  is a coil spring having a small diameter and a spring constant smaller than that of the first spring  64 , and is an elastic member configured to always bias the retainer member  62  toward the positive pressure chamber  601 . One end of the second spring  65  is held on the recessed part  620  of the retainer member  62 . The second spring  65  is provided in a compressed state between an end surface on the negative side in the X-axis direction (bottom part) of the piston  61  and the retainer member  62  (bottom part). 
     The stroke simulator  6  is configured to cause the brake fluid, which has flowed out from the secondary chamber  50 S of the master cylinder  5  through the brake operation by the driver, to flow into an inside of the positive pressure chamber  601  via the positive pressure oil passage  74 , to thereby generate a pedal reaction force. Specifically, when the hydraulic pressure (master cylinder pressure) larger than a predetermined value is applied to a pressure reception surface of the piston  61  in the positive pressure chamber  601 , the piston  61  moves toward the back pressure chamber  602  in the axial direction while compressing the spring  64  and the like. On this occasion, the volume of the positive pressure chamber  601  increases, and, simultaneously, the volume of the back pressure chamber  602  decreases. As a result, the brake fluid flows into the positive pressure chamber  601 . Simultaneously, the brake fluid flows out from the back pressure chamber  602 , and the brake fluid in the back pressure chamber  602  is thus discharged. The back pressure chamber  602  is connected to the back pressure oil passage  16  of the second unit  1 B by the back pressure pipe  10 X. The brake fluid having flowed out from the back pressure chamber  602  through the brake operation by the driver flows through the back pressure pipe  10 X, and is taken into the back pressure oil passage  16  through the back pressure port  874 . In other words, the back pressure pipe  10 X is a pipe configured to take the brake fluid having flowed out from the back pressure chamber  602  into the back pressure oil passage  16 . The stroke simulator  6  is configured to suck the brake fluid from the master cylinder  5  in this way to simulate liquid rigidity of the wheel cylinders W/C, thereby reproducing a sense of stepping on a pedal. When the pressure in the positive pressure chamber  601  falls below the predetermined value, the piston  61  is returned to the initial position by the biasing force (elastic force) of the spring  64  and the like. The damper  66  is configured to come into contact with the retainer member  62 , to thereby be deformed elastically when the piston  61  performs a stroke by an amount equal to or more than a predetermined value. As a result, impact is buffered, and pedal feeling thus increases. 
     Next, detailed description is given of the second unit  1 B. The housing  8  is a block having a generally rectangular parallelepiped shape and being made of aluminum alloy as a material. Outer surfaces of the housing  8  include a front surface  801 , a rear surface  802 , a top surface  803 , a bottom surface  804 , a right side surface  805  and a left side surface  806 . The front surface  801  is a flat surface having a relatively large area. The rear surface  802  is a flat surface approximately parallel with the front surface  801 , and opposes the front surface  801  (across the housing  8 ). The top surface  803  is a flat surface continuing to the front surface  801  and the rear surface  802 . The bottom surface  804  is a flat surface approximately parallel with the top surface  803 , and opposes the top surface  803  (across the housing  8 ). The bottom surface  804  continues to the front surface  801  and the rear surface  802 . The right side surface  805  is a flat surface continuing to the front surface  801 , the rear surface  802 , the top surface  803 , and the bottom surface  804 . The left side surface  806  is a flat surface approximately parallel with the right side surface  805 , and opposes the right side surface  805  (across the housing  8 ). The left side surface  806  is a flat surface continuing to the front surface  801 , the rear surface  802 , the top surface  803 , and the bottom surface  804 . Recessed parts  807  and  808  are formed at corners on the front surface  801  side and the top surface  803  side of the housing  8 . In other words, a corner formed of the front surface  801 , the top surface  803 , and the right side surface  805  and a corner formed of the front surface  801 , the top surface  803 , and the left side surface  806  have cutoff shapes, and thus have the recessed parts  807  and  808 . As viewed in the Y-axis direction, a negative side in the Z-axis direction of the recessed part  807  is approximately orthogonal to an axial center of a cylinder accommodating hole  82 E. A negative side in the Z-axis direction of the recessed part  808  is approximately orthogonal to an axial center of a cylinder accommodating hole  82 A. Positive sides in the Z-axis direction of the recessed parts  807  and  808  are approximately parallel with the Z-axis direction. 
     The front surface  801  is formed on the positive side in the Y-axis direction, and extends in parallel with the X axis and the Z axis. The rear surface  802  is formed on the negative side in the Y-axis direction, and extends in parallel with the X axis and the Z axis. The top surface  803  is formed on the positive side in the Z-axis direction, and extends in parallel with the X axis and the Y axis. The bottom surface  804  is formed on the negative side in the Z-axis direction, and extends in parallel with the X axis and the Y axis. The right side surface  805  is formed on the positive side in the X-axis direction, and extends in parallel with the Y axis and the Z axis. The left side surface  806  is formed on the negative side in the X-axis direction, and extends in parallel with the Y axis and the Z axis. In a state in which the second unit  1 B is mounted to the vehicle, the Z-axis direction is the vertical direction, and the positive side in the Z-axis direction is the top side in the vertical direction. The X-axis direction is the front/rear direction of the vehicle, and the positive side in the X-axis direction is the vehicle rear side. The Y-axis direction is the lateral direction of the vehicle. 
       FIG. 4  to  FIG. 9  are transparent views for illustrating passages, recessed parts, and holes of the housing  8 .  FIG. 4  is a front transparent view for illustrating the housing  8  as viewed from the positive side in the Y-axis direction.  FIG. 5  is a rear transparent view for illustrating the housing  8  as viewed from the negative side in the Y-axis direction.  FIG. 6  is a top transparent view for illustrating the housing  8  as viewed from the positive side in the Z-axis direction.  FIG. 7  is a bottom transparent view for illustrating the housing  8  as viewed from the negative side in the Z-axis direction.  FIG. 8  is a right side transparent view for illustrating the housing  8  as viewed from the positive side in the X-axis direction.  FIG. 9  is a left side transparent view for illustrating the housing  8  as viewed from the negative side in the X-axis direction. The housing  8  includes a cam accommodating hole  81 , the plurality of (five) cylinder accommodating holes  82 A to  82 E, a reservoir chamber  830 , a damper chamber  831 , a liquid reservoir chamber  832 , a plurality of valve body accommodating holes  84 , a plurality of sensor accommodating holes  85 , a power supply hole  86 , a plurality of ports  87 , a plurality of oil passage holes  88 , and a plurality of bolt holes (pin holes)  89 . Those holes and ports are formed by drills or the like. The cam accommodating hole  81  has a bottomed tubular shape extending in the Y-axis direction, and is opened in the front surface  801 . An axial center O of the cam accommodating hole  81  is approximately at a center in the X-axis direction on the front surface  801 , and is present slightly on the negative side in the Z-axis direction with respect to a center in the Z-axis direction. 
     The cylinder accommodating hole  82  has a stepped tubular shape, and extends in a radial direction (radiation direction about the axial center O) of the cam accommodating hole  81 . The cylinder accommodating hole  82  has a small-diameter part  820  on a side closer to the cam accommodating hole  81 , a large-diameter part  821  on a side farther from the cam accommodating hole  81 , and a medium-diameter part  822  between the small-diameter part  820  and the large-diameter part  821 . A part  823  of the medium-diameter part  822  on the side closer to the cam accommodating hole  81  functions as a suction port, and the large-diameter part  821  functions as a discharge port. The cylinder accommodating holes  82  are formed approximately equiangularly (at approximately equal intervals) in a circumferential direction about the axial center O. An angle formed by the axial centers of the cylinder accommodating holes  82  which are adjacent to each other in the circumferential direction of the axial center O is approximately 72° (in a predetermined range including 72°). The plurality of cylinder accommodating holes  82 A to  82 E are arranged in a single row along the Y-axis direction, and are formed on the positive side in the Y-axis direction of the housing  8 . In other words, axial centers of those cylinder accommodating holes  82 A to  82 E are on the same plane a approximately orthogonal to the axial center O. The plane a is approximately in parallel with the front surface  801  and the rear surface  802  of the housing  8 , and is closer to the front surface  801  than to the rear surface  802 . The two cylinder accommodating holes  82 A and  82 E on the positive side in the Z-axis direction are formed on both sides in the X-axis direction with respect to the axial center O The ends on the large diameter part  821  side of the cylinder accommodating holes  82 A and  82 E are opened in the recessed parts  807  and  808 , respectively. The end of the large-diameter part  821  side of the cylinder accommodating hole  82 B is opened in the positive side in the Y-axis direction and on the negative side in the Z-axis direction on the left side surface  806 . The end of the large-diameter part  821  side of the cylinder accommodating hole  82 C is opened approximately at the center in the X-axis direction, and on the positive side in the Y-axis direction on the bottom surface  804 . The cylinder accommodating hole  82 C extends from the bottom surface  804  to the positive side in the Z-axis direction. The end of the large-diameter part  821  side of the cylinder accommodating hole  82 D is opened in the positive side in the Y-axis direction and on the negative side in the Z-axis direction on the right side surface  805 . The small-diameter part  820  of each of the cylinder accommodating holes  82  is opened in an inner peripheral surface of the cam accommodating hole  81 . 
     The reservoir chamber  830  has a bottomed tubular shape, which has an axial center extending in the Z-axis direction, and is opened approximately at a center in the X-axis direction and at a center in the Y-axis direction on the top surface  803 . The reservoir chamber  830  is arranged in a region surrounded by the master cylinder ports  871  and the wheel cylinder ports  872 . (A bottom part on the negative side in the Z-axis direction of) the reservoir chamber  830  is arranged on the positive side in the Z-axis direction with respect to the suction ports  823  of the respective cylinder accommodating holes  82 . The reservoir chamber  830  is formed in a region between the cylinder accommodating holes  82 A and  82 E which are adjacent to each other in the circumferential direction of the axial center O. The cylinder accommodating holes  82 A to  82 E and the reservoir chamber  830  partially overlap with each other in the Y-axis direction (as viewed in the X-axis direction). The damper chamber  831  has a bottomed tubular shape, which has an axial center extending in the Z-axis direction, and is opened approximately at the center in the X-axis direction and slightly on the negative side in the Y-axis direction with respect to the center in the Y-axis direction on the bottom surface  804 . The damper chamber  831  is arranged on the negative side in the Z-axis direction with respect to the cam accommodating hole  81 . The liquid reservoir chamber  832  has a stepped bottomed tubular shape, which has an axial center extending in the Z-axis direction, and is opened on the negative side in the X-axis direction and the positive side in the Y-axis direction in the bottom surface  804 . The liquid reservoir chamber  832  is arranged on the negative side in the Z-axis direction with respect to the cam accommodating hole  81 . The liquid reservoir chamber  832  has a large-diameter part  832   l  on a side closer to the bottom surface  804  (negative side in the Z-axis direction), a small-diameter part  832   s  on a side farther from the bottom surface  804  (positive side in the Z-axis direction), and a medium-diameter part  832   m  between the large-diameter part  832   l  and the small-diameter part  832   s.    
     Each of the plurality of the valve body accommodating holes  84  has a stepped tubular shape, extends in the Y-axis direction, and is opened in the rear surface  802 . The valve body accommodating hole  84  has a large-diameter part  841  on a side closer to the rear surface  802  (negative side in the Y-axis direction), a small-diameter part  84   s  on a side farther from the rear surface  802  (outer side in the positive side in the Y-axis direction), and a medium-diameter part  84   m  between the large-diameter part  841  and the small-diameter part  84   s . The plurality of valve body accommodating holes  84  are arranged in a single row along the Y-axis direction, and are formed on the negative side in the Y-axis direction of the housing  8 . The cylinder accommodating holes  82  and the valve body accommodating holes  84  are arrayed along the Y-axis direction. The plurality of the valve body accommodating holes  84  at least partially overlap with the cylinder accommodating holes  82  as viewed in the Y-axis direction. Most of the plurality of the valve body accommodating holes  84  are contained in a circle connecting the ends on the large-diameter part  821  side (side farther from the axial center O) of the plurality of cylinder accommodating holes  82  to each other. In other words, an outer periphery of this circle and the valve body accommodating holes  84  at least partially overlap with each other. 
     A valve part of the SOL/V OUT  25  is fitted to an SOL/V OUT accommodating hole  845 , and a valve body of the SOL/V OUT  25  is accommodated in the SOL/V OUT accommodating hole  845 . The bypass oil passage  120  and the check valve  220  are formed of, for example, a seal member, which has a cup shape and is provided in the hole  842 . The SOL/V OUT accommodating holes  845   a  to  845   d  are arranged in a single row in the X-axis direction on the positive side in the Z-axis direction of the rear surface  802 . The two SOL/V OUT accommodating holes in the P system are formed on the positive side in the X-axis direction. The two SOL/V OUT accommodating holes in the S system are formed on the negative side in the X-axis direction. In the P system, the hole  845   a  is formed on the positive side in the X-axis direction with respect to the hole  845   d . In the S system, the hole  845   b  is formed on the negative side in the X-axis direction with respect to the hole  845   c . A valve part of the SOL/V IN  22  is fitted to an SOL/V IN accommodating hole  842 , and a valve body of the SOL/V IN  22  is accommodated in the SOL/V IN accommodating hole  842 . The SOL/V IN accommodating holes  842   a  to  842   d  are arranged in a single row in the X-axis direction slightly on the positive side in the Z-axis direction with respect to the axial center O (or at the center in the Z-axis direction of the housing  8 ). The SOL/V IN accommodating hole  842  is adjacent to the SOL/V OUT accommodating hole  845  on the negative side in the Z-axis direction. The two SOL/V IN accommodating holes in the P system are formed on the positive side in the X-axis direction. The two SOL/V IN accommodating holes in the S system are formed on the negative side in the X-axis direction. In the P system, the hole  842   a  is formed on the positive side in the X-axis direction with respect to the hole  842   d . In the S system, the hole  842   b  is formed on the negative side in the X-axis direction with respect to the hole  842   c . The axial centers of the holes  842   a  to  842   d  are approximately at the same positions in the X-axis direction as the axial centers of the holes  845   a  to  845   d , respectively. 
     A valve part of the shutoff valve  21  is fitted to a shutoff valve accommodating hole  841 , and a valve body of the shutoff valve  21  is accommodated in the shutoff valve accommodating hole  841 . The shutoff valve accommodating holes  841 P and  841 S are arrayed in the X-axis direction slightly on the negative side in the Z-axis direction with respect to the center in the Z-axis direction of the housing  8 . The hole  841 P is formed slightly on the positive side in the X-axis direction with respect to a center in the X-axis direction. The hole  841 S is formed slightly on the negative side in the X-axis direction with respect to the center in the X-axis direction. Axial centers of the holes  841 P and  841 S are slightly on the negative side in the Z-axis direction with respect to the axial center O, and are at approximately the same positions in the X-axis direction as the axial centers of the holes  842   d  and  842   c . A valve part of the communication valve  23  is fitted to a communication valve accommodating hole  843 , and a valve body of the communication valve  23  is accommodated in the communication valve accommodating hole  843 . The communication valve accommodating holes  843 P and  843 S are arrayed in the X-axis direction on the negative side in the Z-axis direction with respect to the axial center O. The communication valve accommodating hole  843  is adjacent to the shutoff valve accommodating hole  841  on the negative side in the Z-axis direction. The hole  843 P is formed on the positive side in the X-axis direction with respect to the center in the X-axis direction. The hole  843 S is formed on the negative side in the X-axis direction with respect to the center in the X-axis direction. An axial center of the hole  843 P is slightly on the negative side in the X-axis direction with respect to the axial center of the hole  842   a . An axial center of the hole  843 S is slightly on the positive side in the X-axis direction with respect to the axial center of the hole  842   b . An end on the positive side in the Z-axis direction of the opening of the communication valve accommodating hole  843  overlaps with an end on the negative side in the Z-axis direction of the opening of the shutoff valve accommodating hole  841 , in the Z-axis direction (as viewed in the X-axis direction) on the rear surface  802 . A valve part of the pressure regulating valve  24  is fitted to a pressure regulating valve accommodating hole  844 , and a valve body of the pressure regulating valve  24  is accommodated in the pressure regulating valve accommodating hole  844 . The pressure regulating valve accommodating hole  844  is formed on the negative side in the Z-axis direction with respect to the axial center O, and is formed at approximately the same position in the X-axis direction as the axial center O. The pressure regulating valve accommodating hole  844  is formed between the communication valve accommodating holes  843 P and  843 S in the X-axis direction, and is adjacent to the shutoff valve accommodating holes  841  on the negative side in the Z-axis direction. The pressure regulating valve accommodating hole  844  is at approximately the same position in the Z-axis direction as the communication valve accommodating holes  843 , and is arrayed together with the holes  843 P and  843 S in a single row in the X-axis direction. Both ends in the X-axis direction of the opening of the pressure regulating valve accommodating hole  844  overlap with ends in the X-axis direction of the openings of the shutoff valve accommodating holes  841 , in the X-axis direction (as viewed in the Z-axis direction) on the rear surface  802 . 
     A valve part of the SS/V IN  27  is fitted to an SS/V IN accommodating hole  847 , and a valve body of the SS/V IN  27  is accommodated in the SS/V IN accommodating hole  847 . The bypass oil passage  170  and the check valve  270  are each formed of, for example, a seal member, which has a cup shape and is provided in the hole  847 . A valve part of the SS/V OUT  28  is fitted to an SS/V OUT accommodating hole  848 , and a valve body of the SS/V OUT  28  is accommodated in the SS/V OUT accommodating hole  848 . The bypass oil passage  180  and the check valve  280  are formed of a seal member, which has a cup shape and is provided in the hole  848 . The holes  847  and  848  are arrayed in the X-axis direction on the negative side in the Z-axis direction with respect to the axial center O. The holes  847  and  848  are adjacent to the communication valve accommodating holes  843  and the pressure regulating valve accommodating holes  844  on the negative side in the Z-axis direction. An axial center of the hole  848  is positioned between the axial center of the hole  844  and the axial center of the hole  843 P in the X-axis direction, and is positioned slightly on the positive side in the X-axis direction with respect to an axial center of the hole  841 P. An end on the positive side in the X-axis direction of the opening of the hole  848  overlaps with an end on the negative side in the X-axis direction of the opening of the hole  843 P, in the X-axis direction (as viewed in the Z-axis direction) on the rear surface  802 . An end on the positive side in the Z-axis direction of the opening of the hole  848  overlaps with an end on the negative side in the Z-axis direction of the opening of the hole  843 P, in the Z-axis direction (as viewed in the Y-axis direction). An axial center of the hole  847  is positioned between the axial center of the hole  844  and the axial center of the hole  843 S in the X-axis direction, and is positioned slightly on the negative side in the X-axis direction with respect to an axial center of the hole  841 S. An end on the negative side in the X-axis direction of the opening of the hole  847  overlaps with an end on the positive side in the X-axis direction of the opening of the hole  843 S, in the X-axis direction (as viewed in the Z-axis direction) on the rear surface  802 . An end on the positive side in the Z-axis direction of the opening of the hole  847  overlaps with an end on the negative side in the Z-axis direction of the opening of the hole  843 S, in the Z-axis direction (as viewed in the Y-axis direction). 
     Each of a plurality of sensor accommodating holes  85  has a bottomed tubular shape, which has an axial center extending in the Y-axis direction, and is opened in the rear surface  802 . A pressure sensitive part of the master cylinder pressure sensor  91  is accommodated in a master cylinder pressure sensor accommodating hole  851 . The hole  851  is formed at approximately at the center in the X-axis direction and approximately at the center in the Z-axis direction of the housing  8 , and an axial center of the hole  851  is slightly on the positive side in the Z-axis direction with respect to the axial center O. The holes  851  are formed in a region surrounded by the holes  842 ,  845 ,  841 P, and  841 S. A pressure sensitive part of the discharge pressure sensor  93  is accommodated in a discharge pressure sensor accommodating hole  853 . The hole  853  is formed approximately at the center in the X-axis direction and on the negative side in the Z-axis direction of the housing  8 , and an axial center of the hole  853  is slightly on the negative side in the Z-axis direction with respect to the holes  847  and  848 . The hole  853  is formed in a region surrounded by the holes  844 ,  847 , and  848 . A pressure sensitive part of the wheel cylinder pressure sensor  92  is accommodated in a wheel cylinder pressure sensor accommodating hole  852 . The holes  852 P and  852 S are arrayed in the X-axis direction at approximately the same positions in the Z-axis direction as the axial center O. The hole  852 P is formed on the positive side in the X-axis direction with respect to the center in the X-axis direction. The hole  852 S is formed on the negative side in the X-axis direction with respect to the center in the X-axis direction. An axial center of the hole  852 P is slightly on the positive side in the X-axis direction with respect to the axial center of the hole  842   a . An axial center of the hole  852 S is slightly on the negative side in the X-axis direction with respect to the axial center of the hole  842   b . The hole  852  is formed in a region surrounded by the holes  841 ,  842 , and  843 . The power supply hole  86  has a tubular shape, and passes through the housing  8  (between the front surface  801  and the rear surface  802 ) in the Y-axis direction. The hole  86  is formed approximately at the center in the X-axis direction and on the positive side in the Z-axis direction of the housing  8 . The hole  86  is arranged (formed) in a region surrounded by the holes  842   c  and  842   d  and the holes  845   c  and  845   d , and in a region between the cylinder accommodating holes  82 A and  82 E which are adjacent to each other. 
     Each of the master cylinder ports  871  has a bottomed tubular shape, which has an axial center extending in the Y-axis direction, and is opened in a portion at an end on the positive side in the Z-axis direction between the recessed parts  807  and  808  on the front surface  801 . A primary port  871 P is formed on the positive side in the X-axis direction. The secondary port  871 S is formed on the negative side in the X-axis direction. Both the ports  871 P and  871 S are arrayed in the X-axis direction, and are on both sides of the reservoir chamber  830  and a bolt hole  891  in the X-axis direction (as viewed in the Y-axis direction). The ports  871 P and  871 S are formed respectively between the reservoir chamber  830  and the cylinder accommodating holes  82 A and  82 E in the circumferential direction of the axial center O (as viewed in the Y-axis direction). Openings of the master cylinder ports  871  and an opening of the bolt hole  891  partially overlap with each other in the Z-axis direction (as viewed in the X-axis direction). Each of the wheel cylinder ports  872  has a bottomed tubular shape, which has an axial center extending in the Z-axis direction, and is opened on the negative side in the Y-axis direction (position closer to the rear surface  802  than to the front surface  801 ) in the top surface  803 . The ports  872   a  to  872   d  are arranged in a single row in the X-axis direction. The two ports in the P system are formed on the positive side in the X-axis direction. The two ports in the S system are formed on the negative side in the X-axis direction. In the P system, the port  872   a  is formed on the positive side in the X-axis direction with respect to the port  872   d . In the S system, the port  872   b  is formed on the negative side in the X-axis direction with respect to the port  872   c . The ports  872   c  and  872   d  are on both sides of the suction port  873  (reservoir chamber  830 ) as viewed in the Y-axis direction. An opening of each of the ports  872  and the suction port  873  (opening of the reservoir chamber  830 ) partially overlap with each other in the X-axis direction (as viewed in the Y-axis direction). The opening of each of the ports  872  and an opening of the suction port  873  partially overlap with each other in the Y-axis direction (as viewed in the X-axis direction). 
     The suction port  873  is the opening of the reservoir chamber  830  on the top surface  803 , is formed so as to be directed to the top side in the vertical direction, and is opened on the top side in the vertical direction. The port  873  is opened at a position on a center side in the X-axis direction and on a center side in the Y-axis direction closer to the front surface  801  than the wheel cylinder ports  872 , on the top surface  803 . The port  873  is formed on the positive side in the Z-axis direction with respect to the suction ports  823  of the cylinder accommodating holes  82 A to  82 E. The cylinder accommodating holes  82 A and  82 E are on both sides of the port  873  as viewed in the Y-axis direction. An opening of each of the cylinder accommodating holes  82 A and  82 E and the port  873  partially overlap with each other in the Y-axis direction (as viewed in the X-axis direction). The back pressure port  874  has a bottomed tubular shape, which has an axial center extending in the X-axis direction, and is opened slightly on the negative side in the Y-axis direction and on the negative side in the Z-axis direction with respect to the axial center O on the right side surface  805 . The axial center of the port  874  is positioned between an axial center of the communication valve accommodating hole  843  and an axial center of the SS/V OUT accommodating hole  848  in the Z-axis direction. 
     The plurality of oil holes  88  include first to fifth hole groups  88 - 1  to  88 - 5  and oil passage holes  880  and  881 . The first hole group  88 - 1  connects the master cylinder ports  871 , the shutoff valve accommodating holes  841 , and the master cylinder pressure sensor accommodating hole  851  to one another. The second hole group  88 - 2  connects the shutoff valve accommodating holes  841 , the communication valve accommodating holes  843 , the SOL/V IN accommodating holes  842 , the SS/V IN accommodating hole  847 , and the wheel cylinder pressure sensor accommodating holes  852  to one another. The third hole group  88 - 3  connects the discharge ports  821  of the cylinder accommodating holes  82 , the communication valve accommodating holes  843 , the pressure regulating valve accommodating holes  844 , and the discharge pressure sensor accommodating hole  853  to one another. The fourth hole group  88 - 4  connects the reservoir chamber  830 , the suction ports  823  of the cylinder accommodating holes  82 , the SOL/V OUT accommodating holes  845 , the SS/V OUT accommodating hole  848 , and the pressure regulating valve accommodating hole  844  to one another. The fifth hole group  88 - 5  connects the back pressure port  874 , the SS/V IN accommodating hole  847 , and the SS/V OUT accommodating hole  848  to one another. Each of the oil holes  880  connects the SOL/V IN accommodating hole  842  and the wheel cylinder port  872  to each other. The oil passage hole  881  connects the cam accommodating hole  81  and the liquid reservoir chamber  832  to each other. 
     The first hole group  88 - 1  includes first holes  88 - 11  to seventh holes  88 - 17 . First, description is given of the P system. The first hole  88 - 11 P extends from a bottom part of the primary port  871 P to the negative side in the Y-axis direction. The second hole  88 - 12 P extends from the right side surface  805  to the negative side in the X-axis direction, and is connected to the first hole  88 - 11 P. The third hole  88 - 13 P extends from the rear surface  802  to the positive side in the Y-axis direction, and is connected to the second hole  88 - 12 P. The fourth hole  88 - 14 P extends from the positive side in the Y-axis direction of the third hole  88 - 13 P to the negative side in the Z-axis direction. The fifth hole  88 - 15 P extends from the rear surface  802  to the positive side in the Y-axis direction, and is connected to the fourth hole  88 - 14 P. The sixth hole  88 - 16 P extends from an end on the positive side in the Y-axis direction of the fifth hole  88 - 15 P to the positive side in the X-axis direction, the negative side in the Y-axis direction, and the negative side in the Z-axis direction, and is connected to the medium-diameter part  84   m  of the shutoff valve accommodating hole  841 P. The seventh hole  88 - 17  extends from the left side surface  806  to the positive side in the X-axis direction, is connected to the fifth hole  88 - 15 P, and is connected to the master cylinder pressure sensor accommodating hole  851 . The S system is symmetrical with the P system about the center in the X-axis direction of the housing  8  except that the seventh hole  88 - 17  is not included. 
     The second hole group  88 - 2  includes first holes  88 - 21  to seventh holes  88 - 27 . First, description is given of the P system. The first hole  88 - 21 P extends over a short distance from a bottom part of the shutoff valve accommodating holes  841  to the positive side in the Y-axis direction. The second hole  88 - 22 P extends from the right side surface  805  to the negative side in the X-axis direction, and is connected to the first hole  88 - 21 P. The third hole  88 - 23 P extends from the top surface  803  to the negative side in the Z-axis direction, and is connected to the second hole  88 - 22 P on the positive side in the X-axis direction. The fourth hole  88 - 24 P extends from the right side surface  805  to the negative side in the X-axis direction, and is connected to an intermediate portion of the third hole  88 - 23 P. The fifth holes  88 - 25   a  and  88 - 25   d  extend over short distances from the positive side in the X-axis direction of the fourth hole  88 - 24 P to the positive side in the Y-axis direction, and are connected to bottom parts of the SOL/V IN accommodating holes  842   a  and  842   d , respectively. The sixth hole  88 - 26 P extends from an intermediate portion of the second hole  88 - 22 P to the negative side in the Y-axis direction and the negative side in the Z-axis direction, and is connected to the medium-diameter part  84   m  of the communication valve accommodating hole  843 P. The seventh hole  88 - 27 P extends from a bottom part of the wheel cylinder pressure sensor accommodating hole  852 P to the positive side in the Y-axis direction, and is connected to an intermediate portion of the second hole  88 - 22 P. The S system is symmetrical with the P system about the center in the X-axis direction of the housing  8  except that the eighth hole  88 - 28  is included. The eighth hole  88 - 28  extends from the negative side in the X-axis direction of the bottom surface  804  to the positive side in the Z-axis direction, is connected to the medium-diameter part  84   m  of the SS/V IN accommodating hole  847 , and is connected to the medium-diameter part  84   m  of the communication valve accommodating hole  843 S. 
     The third hole group  88 - 3  includes a first hole  88 - 31  to a twelfth hole  88 - 312 . The first hole  88 - 31  extends from the discharge port  821  of the cylinder accommodating hole  82 A to the negative side in the Z-axis direction. The second hole  88 - 32  extends from an end of the first hole  88 - 31  to the negative side in the X-axis direction and the negative side in the Z-axis direction, and is connected to the discharge port  821  of the cylinder accommodating hole  82 B. The third hole  88 - 33  extends from the discharge port  821  of the cylinder accommodating hole  82 B to the positive side in the X-axis direction and the negative side in the Z-axis direction. The fourth hole  88 - 34  extends from an end of the third hole  88 - 33  to the positive side in the X-axis direction and the negative side in the Z-axis direction, and is connected to the discharge port  821  of the cylinder accommodating hole  82 C. The fifth hole  88 - 35  extends from the discharge port  821  of the cylinder accommodating hole  82 C to the positive side in the X-axis direction and the positive side in the Z-axis direction. The sixth hole  88 - 36  extends from an end of the fifth hole  88 - 35  to the positive side in the X-axis direction and the positive side in the Z-axis direction, and is connected to the discharge port  821  of the cylinder accommodating hole  82 D. The seventh hole  88 - 37  extends from the discharge port  821  of the cylinder accommodating hole  82 D to the negative side in the X-axis direction and the positive side in the Z-axis direction. The eighth hole  88 - 38  extends from an end of the seventh hole  88 - 37  to the positive side in the Z-axis direction, and is connected to the discharge port  821  of the cylinder accommodating hole  82 E. The ninth hole  88 - 39  extends from a bottom part of the discharge pressure sensor accommodating hole  853  to the positive side in the Y-axis direction, is connected to the damper chamber  831 , and is connected to the discharge port  821  of the cylinder accommodating hole  82 C. The tenth hole  88 - 310  extends from a bottom part of the damper chamber  831  to the positive side in the Z-axis direction. The eleventh hole  88 - 311  extends from the right side surface  805  to the negative side in the X-axis direction, is connected to bottom parts of both of the communication valve accommodating holes  843 , and is connected to an end of the tenth hole  88 - 310 . The twelfth hole  88 - 312  (not shown) extends over a short distance from a bottom part of the pressure regulating valve accommodating hole  844  to the positive side in the Y-axis direction, and is connected to the eleventh hole  88 - 311 . 
     The fourth hole group  88 - 4  includes a first hole  88 - 41  to a ninth hole  88 - 49 . The first hole  88 - 41  extends from the left side surface  806  to the positive side in the X-axis direction, is connected to a bottom part of the reservoir chamber  830 , and is connected to bottom parts of the SOL/V OUT accommodating holes  845 . The second hole  88 - 42  extends from the bottom part of the reservoir chamber  830  to the positive side in the X-axis direction, the positive side in the Y-axis direction, and the negative side in the Z-axis direction, and is connected to the suction port  823  of the cylinder accommodating hole  82 A. The third hole  88 - 43  extends from the bottom part of the reservoir chamber  830  to the positive side in the X-axis direction, the positive side in the Y-axis direction, and the negative side in the Z-axis direction, and is connected to the suction port  823  of the cylinder accommodating hole  82 E. The fourth hole  88 - 44  extends from the left side surface  806  to the positive side in the X-axis direction, and is connected to the suction port  823  of the cylinder accommodating hole  82 A. The fifth hole  88 - 45  extends from the right side surface  805  to the negative side in the X-axis direction, and is connected to the suction port  823  of the cylinder accommodating hole  82 E. The sixth hole  88 - 46  extends from a bottom part of the liquid reservoir chamber  832  to the positive side in the Z-axis direction, is connected to the suction port  823  of the cylinder accommodating hole  82 B, and is connected to an intermediate portion of the fourth hole  88 - 44 . The seventh hole  88 - 47  extends from the bottom surface  804  to the positive side in the Z-axis direction, is connected to the suction port  823  of the cylinder accommodating hole  82 D, and is connected to an intermediate portion of the fifth hole  88 - 45 . The eighth hole  88 - 48  extends from the right side surface  805  to the negative side in the X-axis direction and the positive side in the Z-axis direction, is connected to the suction port  823  of the cylinder accommodating hole  82 C, and is connected to an intermediate portion of the sixth hole  88 - 46  and an intermediate portion of the seventh hole  88 - 47 . The ninth hole  88 - 49  extends from a bottom part of the SS/V OUT accommodating hole  848  to the positive side in the Y-axis direction, and is connected to an intermediate portion of the seventh hole  88 - 47 . 
     The fifth hole group  88 - 5  includes a first hole  88 - 51  to a sixth hole  88 - 56 . The first hole  88 - 51  extends from a bottom part of the back pressure port  874  to the negative side in the X-axis direction. The second hole  88 - 52  extends from an end of the first hole  88 - 51  to the negative side in the Z-axis direction. The third hole  88 - 53  extends from the rear surface  802  to the positive side in the Y-axis direction. The third hole  88 - 53  is connected to the second hole  88 - 52  in the course. The fourth hole  88 - 54  extends from the left surface  806  to the positive side in the X-axis direction. An end of the third hole  88 - 53  is connected to an intermediate portion of the fourth hole  88 - 54 . The fifth hole  88 - 55  extends from an end of the fourth hole  88 - 54  to the negative side in the Y-axis direction over a short distance, and is connected to a bottom part of the SS/V IN accommodating hole  847 . The sixth hole  88 - 56  extends from an intermediate portion of the first hole  88 - 51  to the negative side in the Y-axis direction and the negative side in the Z-axis direction over a short distance, and is connected to the medium-diameter part  84   m  of the SS/V OUT accommodating hole  848 . Each of the holes  880  extends from a bottom part of the wheel cylinder port  872  to the negative side in the Z-axis direction, is connected to the medium-diameter part  84   m  of the SOL/V OUT accommodating hole  845 , and is connected to the medium-diameter part  84   m  of the SOL/V IN accommodating hole  842 . The hole  881  extends from the cam accommodating hole  81  to the negative side in the X-axis direction and the negative side in the Z-axis direction, and is connected to the medium-diameter part  832   m  of the liquid reservoir chamber  832 . 
     The first hole  88 - 11  to the sixth hole  88 - 16 P of the first hole group  88 - 1  connect the master cylinder ports  871  and the shutoff valve accommodating holes  841  to each other, and function as a part of the supply oil passages  11 . The first hole  88 - 21  to the fifth hole  88 - 25  of the second hole group  88 - 2  connect the shutoff valve accommodating holes  841  and the SOL/V IN accommodating holes  842  to each other, and function as a part of the supply oil passages  11 . The sixth hole  88 - 26 P connects the communication valve accommodating hole  843  and the second hole  88 - 22 P to each other, and functions as a part of the discharge oil passage  13 . The eighth hole  88 - 28  connects the SS/V IN accommodating hole  847  and the communication valve accommodating hole  843 S to each other, and functions as a part of the first simulator oil passage  17 . Each of the holes  880  connects the SOL/V IN accommodating hole  842  and the wheel cylinder port  872  to each other, and functions as a part of the supply oil passage  11 . Moreover, each of the holes  880  connects the SOL/V IN accommodating hole  842  and the SOL/V OUT accommodating hole  845  to each other, and functions as a part of the pressure reducing oil passage  15 . The first hole  88 - 31  to the eleventh hole  88 - 311  of the third hole group  88 - 3  connect the discharge ports  821  of the cylinder accommodating holes  82  and the communication valve accommodating holes  843  to each other, and function as a part of the discharge oil passages  13 . The twelfth hole  88 - 312  connects the eleventh hole  88 - 311  and the pressure regulating valve accommodating hole  844  to each other, and functions as a part of the pressure regulating oil passage  14 . The first hole  88 - 41  of the fourth hole group  88 - 4  connects the SOL/V OUT accommodating hole  845  and the reservoir chamber  830  to each other, and functions as a part of the pressure reducing oil passage  15 . The second hole  88 - 42  to the eighth hole  88 - 48  connect the reservoir chamber  830  and the suction ports  823  of the cylinder accommodating holes  82  to each other, and function as the suction oil passage  12 . The ninth hole  88 - 49  connects the SS/V OUT accommodating hole  848  and the seventh hole  88 - 47  to each other, and functions as the second simulator oil passage  18 . The first hole  88 - 51  to the fifth hole  88 - 55  of the fifth hole group  88 - 5  connect the back pressure port  874  and the SS/V IN accommodating hole  847  to each other, and function as a part of the back pressure oil passage  16  and the first simulator oil passages  17 . The sixth hole  88 - 56  connects the first hole  88 - 51  and the SS/V OUT accommodating hole  848  to each other, and functions as a part of the second simulator oil passage  18 . The hole  881  connects the cam accommodating hole  81  and the liquid reservoir chamber  832  to each other, and serves as a drain oil passage. 
     A plurality of bolt holes  89  include bolt holes  891  to  895 . The bolt hole  891  has a bottomed tubular shape, which has an axial center extending in the Y-axis direction, and is opened in the front surface  801 . Three holes  891  are formed at positions approximately symmetrical with respect to the axial center O of the cam accommodating hole  81 . Distances from the axial center O to the respective holes  891  are approximately the same. One hole  891  is formed approximately at the center in the X-axis direction (position overlapping with the axial center O in the X-axis direction) and on the positive side in the Z-axis direction with respect to the axial center O in the front surface  801 . This hole  891  is positioned between the master cylinder ports  871 P and  871 S in the X-axis direction, and overlaps with the reservoir chamber  830  as viewed in the Y-axis direction. The other two holes  891  are on both sides in the X-axis direction with respect to the axial center O, and on the negative side in the Z-axis direction with respect to the axial center O. The bolt hole  892  has a bottomed tubular shape, which has an axial center extending in the Y-axis direction, and is opened in the rear surface  802 . A total of four holes  892  are formed at four corners of the rear surface  802 , respectively. The bolt hole  893  has a bottomed tubular shape, which has an axial center extending in the Z-axis direction, and is opened in the top surface  803 . One hole  893  is formed approximately at the center in the X-axis direction (position overlapping with the axial center O in the X-axis direction) on the positive side in the Y-axis direction in the top surface  803 . The bolt hole  894  has a bottomed tubular shape, which has an axial center extending in the Y-axis direction, and is opened in the front surface  801 . Two holes  894  are formed on the negative side in the Z-axis direction with respect to the axial center O and at both ends in the X-axis direction in the front surface  801 . The holes  894  are positioned on an opposite side of the master cylinder port  871  with respect to the axial center O. The hole  894  on the negative side in the X-axis direction is approximately on the opposite side of the primary port  871 P with respect to the axial center O. The hole  894  on the positive side in the X-axis direction is approximately on the opposite side of the secondary port  871 S with respect to the axial center O. The axial centers of the holes  894  are arranged on the negative side in the Z-axis direction with respect to the axial centers of the bolt holes  891  on the negative side in the Z-axis direction, and on sides (outer sides) closer to the side surfaces  805  and  806  in the X-axis direction. The bolt hole  895  has a bottomed tubular shape, which has an axial center extending in the Z-axis direction. Two bolt holes  895  are provided, and are opened approximately at the center in the Y-axis direction, and on both ends in the X-axis direction on the bottom surface  804 . An end on the positive side in the Z-axis direction of the hole  895  overlaps with the bolt hole  894  as viewed in the Y-axis direction. 
     (Mount Fixation) 
       FIG. 10  is a front view for illustrating the second unit  1 B as viewed from the positive side in the Y-axis direction.  FIG. 11  is a rear view for illustrating the second unit  1 B as viewed from the negative side in the Y-axis direction.  FIG. 12  is a right side view for illustrating the second unit  1 B as viewed from the positive side in the X-axis direction.  FIG. 13  is a left side view of the second unit  1 B as viewed from the negative side in the X-axis direction.  FIG. 14  is a top view for illustrating the second unit  1 B as viewed from the positive side in the Z-axis direction. A mount  102  is a pedestal formed by bending a metal plate, and is fixed by fastening bolts to the vehicle body side (a bottom surface of the motor room). The mount  102  integrally includes a first mount part  102   a , a second mount part  102   b , and leg parts  102   c  to  102   h . The first mount part  102   a  is arranged approximately in parallel with the X axis and the Y axis. Bolt holes are formed at an end on the negative side in the Y-axis direction at ends on both sides in the X-axis direction of the first mount part  102   a . Bolts B 3  are inserted into those bolt holes from the negative side in the Z-axis direction. The second mount part  102   b  extends from an end on the positive side in the Y-axis direction of the first mount part  102   a  to the positive side in the Z-axis direction. An end on the positive side in the Z-axis direction of the second mount part  102   b  curves to form a recessed shape along a tubular part  201  of a motor housing  200 . Bolt holes are formed at an end on the positive side in the Z-axis direction at ends on both sides in the X-axis direction of the second mount part  102   b . Bolts B 4  are inserted into those bolt holes from the positive side in the Y-axis direction. 
     The leg part  102   c  extends from an end on the negative side in the Y-axis direction of the first mount part  102   a  to the negative side in the Z-axis direction. The leg part  102   d  extends from an end on the negative side in the X-axis direction of the first mount part  102   a  to the negative side in the Z-axis direction. The leg part  102   e  extends from an end on the positive side in the X-axis direction of the first mount part  102   a  to the negative side in the Z-axis direction. The leg part  102   f  extends from an end on the negative side in the Z-axis direction of the leg part  102   c  to the negative side in the Y-axis direction. A plurality of bolt holes are arranged in a row in the X-axis direction in the leg part  102   f . Bolts configured to fix the mount  102  to the vehicle body side are inserted into those bolt holes from the positive side in the Z-axis direction. The leg part  102   g  extends from an end on the negative side in the Z-axis direction of the leg part  102   d  to the negative side in the X-axis direction. A plurality of bolt holes are arranged in a row in the Y-axis direction in the leg part  102   g . Bolts configured to fix the mount  102  to the vehicle body side are inserted into those bolt holes from the positive side in the Z-axis direction. The leg part  102   h  extends from an end on the negative side in the Z-axis direction of the leg part  102   e  to the positive side in the X-axis direction. A plurality of bolt holes are arranged in a row in the Y-axis direction in the leg part  102   h . Bolts configured to fix the mount  102  to the vehicle body side are inserted into those bolt holes from the positive side in the Z-axis direction. The bolts B 3  of the first mount part  102   a  are inserted into the bolt holes  895  of the housing  8 , and are fixed. The bolts B 3  are configured to fix the bottom surface  804  of the housing  8  to the first mount part  102   a  via an insulator  103 . The bolts B 4  of the second mount part  102   b  are inserted into the bolt holes  894  of the housing  8 , and are fixed. The bolts B 4  are configured to fix the front surface  801  of the housing  8  to the second mount part  102   b  via an insulator  104 . The bolt holes  894  and  895  function as fixing holes (fixing parts) for fixing the housing  8  to the vehicle body side (mount  102 ). The insulators  103  and  104  are elastic members configured to suppress (insulate) vibration. 
     (Port Connection) 
     Each of the ports  871  to  874  continues to the oil passage inside the housing  8 , and connects the oil passage inside and an oil passage (pipe  10 M or the like) outside the housing  8  to each other. The master cylinder ports  871  are ports configured to connect the housing  8  (second unit  1 B) to the master cylinder  5  (hydraulic pressure chambers  50 ). The master cylinder ports  871  are connected to the supply oil passages  11  inside the housing  8 , and are connected to (the pipes  10 M from) the master cylinder  5  outside the housing  8 . The master cylinder ports  871  are formed on the positive side in the Z-axis direction (the top side in the vertical direction) with respect to the axial center O, and on the positive side in the Z-axis direction of the motor  20  (motor housing  200 ). The other end of the primary pipe  10 MP is fixedly provided in the primary port  871 P (the primary pipe  10 MP is mounted and connected). The other end of the secondary pipe  10 MS is fixedly provided in the secondary port  871 S (the secondary pipe  10 MS is mounted and connected). The wheel cylinder ports  872  are ports configured to connect the housing  8  (second unit  1 B) to the wheel cylinders W/C. The wheel cylinder ports  872  are connected to the supply oil passages  11  inside the housing  8 , and are connected to (the pipes  10 W from) the wheel cylinders W/C outside the housing  8 . The other end of each of the wheel cylinder pipes  10 W is fixedly provided in each of the wheel cylinder ports  872  (the wheel cylinder pipe  10 W is mounted and connected). 
     The suction port  873  is a port (connection port) configured to connect the housing  8  (second unit  1 B) to the reservoir tank  4 . The suction port  873  is connected to the reservoir chamber  830  inside the housing  8 , and are connected to (the pipe  10 R from) the reservoir tank  4  outside the housing  8 . The nipple  10 R 2  is fixedly provided in the suction port  873 , and the other end of the suction pipe  10 R is connected to the nipple  10 R 2 . The bolt hole  893  functions as a fixing hole (fixing part) for fixing the nipple  10 R 2  to the housing  8 . The back pressure port  874  is a port configured to connect the housing  8  (second unit  1 B) to the stroke simulator  6  (back pressure chamber  602 ). The back pressure port  874  is connected to the back pressure oil passage  16  inside the housing  8 , and is connected to (the pipe  10 X from) the stroke simulator  6  outside the housing  8 . The other end of the back pressure pipe  10 X is fixedly provided in the back pressure port  874  (the back pressure pipe  10 X is mounted and connected). 
     (Motor Fixation) 
     The motor  20  is arranged on the front surface  801  of the housing  8 , and the motor housing  200  is mounted thereto. The front surface  801  functions as a motor mounting surface. The bolt holes  891  function as fixing holes (fixing parts) configured to fix the motor  20  to the housing  8 . The motor  20  includes the motor housing  200 . The motor housing  200  has a bottomed tubular shape, and includes a tubular part  201 , a bottom part  202 , and a flange part  203 . The tubular part  201  accommodates a stator, a rotor, and the like on its inner peripheral side. A rotation shaft of the motor  20  extends on an axial center of the tubular part  201 . The bottom part  202  closes one side in the axial direction of the tubular part  201 . The flange part  203  is provided at an end on the other side (opening side) in the axial direction of the tubular part  201 , and extends from an outer peripheral surface of the tubular part  201  to a radially outer side. The flange part  203  includes first, second, and third protruded parts  203   a ,  203   b , and  203   c . A bolt hole passes through each of the protruded parts  203   a  to  203   c . A bolt b 1  is inserted into each of the bolt holes. The bolt b 1  is fastened to the bolt hole  891  of the housing  8 . The flange part  203  is fastened to the front surface  801  with bolts b 1 . Conductive members (power supply connector) for current supply is connected to the stator. The conductive members are integrated together with wires configured to transmit a detection signal of a resolver. The conductive members extending from the stator are accommodated (mounted) in the power supply hole  86 , and protrude from the rear surface  802  to the negative side in the Y-axis direction. The power supply hole  86  functions as a mounting hole in which the conductive members are mounted. 
       FIG. 15  is a cross-sectional view for illustrating the second unit  1 B taken along the plane a, and is a cross-sectional view as viewed in a direction indicated by XV-XV of  FIG. 14 . The axial center (axis) of the rotation shaft of the motor  20  approximately matches the axial center O of the cam accommodating hole  81 . A rotation shaft (hereinafter referred to as “pump rotation shaft”)  300  of the pump and a cam unit  30  are accommodated in the cam accommodating hole  81 . The pump rotation shaft  300  is a drive shaft of the pump  3 . The pump rotation shaft  300  is fixed to the rotation shaft of the motor  20  so that the axial center thereof extends on an extension of the axial center of the rotation shaft of the motor  20 , and is rotationally driven by the motor  20 . The axial center of the pump rotation shaft  300  approximately matches the axial center O. The pump rotation shaft  300  rotates integrally with the rotation shaft of the motor  20  about the axial center O. The cam unit  30  is provided on the pump rotation shaft  300 . The cam unit  30  includes a cam  301 , a drive member  302 , and a plurality of rolling elements  303 . The cam  301  is an eccentric cam having a cylindrical shape, and has an axial center P eccentric with respect to the axial center O of the pump rotation shaft  300 . The axial center P extends approximately in parallel with the axial center O. The cam  301  oscillates while rotating about the axial center O integrally with the pump rotation shaft  300 . The drive member  302  has a tubular shape, and is arranged on an outer peripheral side of the cam  301 . An axial center of the drive member  302  approximately matches the axial center P. The drive member  302  can rotate about the axial center P with respect to the cam  301 . The drive member  302  has the same structure as that of an outer race of a roller bearing. The plurality of rolling elements  303  are arranged between an outer peripheral surface of the cam  301  and an inner peripheral surface of the Drive member  302 . The rolling element  303  is a needle roller, and extends along the axial center direction of the pump rotation shaft  300 . 
     The pump  3  includes the housing  8 , the pump rotation shaft  300 , a cam unit  30 , and the plurality of (five) pump parts  3 A to  3 E. Each of the pump parts  3 A to  3 E is a piston pump (reciprocating pump), and is configured to suck and discharge the brake fluid as working fluid as a result of a reciprocating motion of the piston (plunger)  36 . The cam unit  30  has a function of converting the rotational motion of the pump rotation shaft  300  to the reciprocating motions of the pistons  36 . Hereinafter, when components of the respective pump parts  3 A to  3 E are distinguished from one another, suffixes A to E are added to reference symbols. The respective pistons  36  are arranged around the cam unit  3 M, and are respectively accommodated in the cylinder accommodating holes  82 . An axial center  360  of the piston  36  approximately matches the axial center of the cylinder accommodating hole  82 , and extends in a radial direction of the pump rotation shaft  300 . In other words, the number of pistons  36  is equal to the number (five) of the cylinder accommodating holes  82 , and the pistons  36  extend in the radiation directions with respect to the axial center O. The pistons  36 A to  36 E are arranged approximately equiangularly in a circumferential direction of the pump rotation shaft  300  (hereinafter simply referred to as “circumferential direction”), in other words, at approximately equal intervals in a rotation direction of the pump rotation shaft  300 . The axial centers  360 A to  360 E of those pistons  36 A to  36 E are on the same plane a. Those pistons  36 A to  36 E are driven by the same pump rotation shaft  300  and the same cam unit  30 . 
     Each of the pump parts  3 A to  3 E includes a cylinder sleeve  31 , a filter member  32 , a plug member  33 , a guide ring  34 , a first seal ring  351 , a second seal ring  352 , the piston  36 , a return spring  37 , a suction valve  38 , and a discharge valve  39 , and those components are provided in the cylinder accommodating hole  82 . The cylinder sleeve  31  has a bottomed tubular shape, and a hole  311  passes through a bottom part  310 . The cylinder sleeve  31  is fixed in the cylinder accommodating hole  82 . An axial center of the cylinder sleeve  31  approximately matches the axial center  360  of the cylinder accommodating hole  82 . An end  312  on an opening side of the cylinder sleeve  31  is arranged in the medium-diameter part  822  (suction port  823 ), and the bottom part  310  is arranged in the large-diameter part (discharge port)  821 . The filter member  32  has a bottomed tubular shape. A hole  321  passes through a bottom part  320 , and a plurality of openings pass through a sidewall part. Filters are provided in the openings. An end  323  on an opening side of the filter member  32  is fixed to the end part  312  on the opening side of the cylinder sleeve  31 . The bottom part  320  is arranged in the small-diameter part  820 . An axial center of the filter member  32  approximately matches the axial center  360  of the cylinder accommodating hole  82 . There is a gap between an outer peripheral surface on which the openings of the filter member  32  open and an inner peripheral surface of the cylinder accommodating hole  82  (suction port  823 ). The passages (the oil passage  88 - 42  and the like) on the suction side communicate with the suction port  823  and the gap. The plug member  33  has a cylindrical shape, and includes a recessed part  330  and a groove (not shown) on one end side in the axial center direction. This groove extends in the radial direction, connects the recessed part  330  and an outer peripheral surface of the plug member  33  to each other, and communicates with the discharge port  821 . One end side in the axial direction of the plug member  33  is fixed to the bottom part  310  of the cylinder sleeve  31 . An axial center of the plug member  33  approximately matches the axial center  360  of the cylinder accommodating hole  82 . The plug member  33  is fixed to the large-diameter part  821 , and closes the opening of the cylinder accommodating hole  82  on an outer peripheral surface of the housing  8 . The passages (the oil passage  88 - 31  and the like) on the discharge side communicate with the discharge port  821  and the groove of the plug member  33 . The guide ring  34  has a tubular shape, and fixed to a side (small-diameter part  820 ) closer to the cam accommodating hole  81  than the filter member  32  in the cylinder accommodating hole  82 . An axial center of the guide ring  34  approximately matches the axial center  360  of the cylinder accommodating hole  82 . The first seal ring  351  is provided between the guide ring  34  and the filter member  32  in the cylinder accommodating hole  82  (small-diameter part  820 ). 
     The piston  36  has a cylindrical shape, has an end surface (hereinafter referred to as “piston end surface”)  361  on one side in an axial center direction, and a flange part  362  on an outer periphery on the other side in the axial center direction. The piston end surface  361  has a flat surface shape extending in a direction approximately orthogonal to the axial center  360  of the piston  36 , and has an approximately circular shape with the axial center  360  as a center. The piston  36  has an axial hole  363  and a radial hole  364 . The axial hole  363  extends on the axial center  360 , and is opened in an end surface on the other side in the axial center direction of the piston  36 . The radial hole  364  extends in the radial direction of the piston  36 , is opened in the outer peripheral surface on the one side in the axial center direction with respect to the flange part  362 , and is connected to the one side in the axial center direction of the axial hole  363 . A check valve case  365  is fixed to an end on the other side in the axial center direction of the piston  36 . The check valve case  365  is formed of a thin plate having a bottomed tubular shape, and includes a flange part  366  on an outer periphery of an end on an opening side, and a plurality of holes  368  pass through a sidewall part and a bottom part  367 . The end on the opening side of the check valve case  365  is fitted to an end on the other side in the axial center direction of the piston  36 . The second seal ring  352  is provided between the flange part  366  of the check valve case  365  and the flange part  362  of the piston  36 . The other side in the axial center direction of the piston  36  is inserted onto an inner peripheral side of the cylinder sleeve  31 , and the flange part  362  is thus guided and supported by the cylinder sleeve  31 . The one side in the axial center direction of the piston  36  with respect to the radial hole  364  is inserted onto an inner peripheral side (hole  321 ) of the bottom part  320  of the filter member  32 , an inner peripheral side of the first seal ring  351 , and an inner peripheral side of the guide ring  34 , and is guided and supported thereby. The axial center  360  of the piston  36  approximately matches the axial centers of the cylinder sleeve  31  and the like (cylinder accommodating hole  82 ). The end (piston end surface  361 ) on the one end side in the axial center direction of the piston  36  protrudes into the cam accommodating hole  81 . 
     The return spring  37  is a compression spring, and is provided on the inner peripheral side of the cylinder sleeve  31 . One end of the return spring  37  is provided in the bottom part  310  of the cylinder sleeve  31 , and the other end is provided in the flange part  366  of the check valve case  365 . The return spring  37  is configured to always bias the piston  36  to the cam accommodating hole  81  side with respect to the cylinder sleeve  31  (cylinder accommodating hole  82 ). The suction valve  38  includes a ball  380  as a valve body and a return spring  381 , and the ball  380  and the return spring  381  are accommodated on an inner peripheral side of the check valve case  365 . A valve seat  369  is provided around an opening of the axial hole  363  on the end surface on the other side in the axial center direction of the piston  36 . The axial hole  363  is closed by the ball  380  seating on the valve seat  369 . The return spring  381  is a compression coil spring, one end thereof is provided in the bottom part  367  of the check valve case  365 , and the other end is provided on the ball  380 . The return spring  381  is configured to always bias the ball  380  toward the valve seat  369  side with respect to the check valve case  365  (piston  36 ). The discharge valve  39  includes a ball  390  as a valve body and a return spring  391 , and the ball  390  and the return spring  391  are accommodated in a recessed part  330  of the plug member  33 . A valve seat  313  is provided around an opening of the through hole  311  in the bottom part  310  of the cylinder sleeve  31 . The through hole  311  is closed by the ball  390  seating on the valve seat  313 . The return spring  391  is a compression coil spring, one end thereof is provided in a bottom surface of the recessed part  330 , and the other end is provided on the ball  390 . The return spring  391  is configured to always bias the ball  390  toward the valve seat  313  side. 
     A space R 1  on the cam accommodating hole  81  side with respect to the flange part  362  of the piston  36  inside the cylinder accommodating hole  82  is a space on the suction side communicating with the suction oil passage  12  in the housing  8 . Specifically, a space from the gap between the outer peripheral surface of the filter member  32  and the inner peripheral surface (suction port  823 ) of the cylinder accommodating hole  82  to the radial hole  364  and the axial hole  363  of the piston  36  via the plurality of openings of the filter member  32  and a gap between an outer peripheral surface of the piston  36  and an inner peripheral surface of the filter member  32  functions as the suction-side space R 1 . Communication of this suction-side space R 1  with the cam accommodating hole  81  is suppressed by the first seal ring  351 . A space R 3  between the cylinder sleeve  31  and the plug member  33  inside the cylinder accommodating hole  82  is a space on the discharge side communicating with the discharge oil passage  13  in the housing  8 . Specifically, a space from the groove of the plug member  33  to the discharge port  821  functions as the discharge-side space R 3 . The volume of a space R 2  between the flange part  362  of the piston  36  and the bottom part  310  of the cylinder sleeve  31  on the inner peripheral side of the cylinder sleeve  31  changes through a reciprocating motion (stroke) of the piston  36  with respect to the cylinder sleeve  31 . This space R 2  communicates with the suction-side space R 1  through the opening of the suction valve  38  and the discharge-side space R 3  through the opening of the discharge valve  39 . 
     The piston  36  of each of the pump parts  3 A to  3 E reciprocates to provide a pump action. In other words, when the piston  36  performs a stroke toward the side approaching the cam accommodating hole  81  (axial center  510 ), the volume of the space R 2  increases, and the pressure in R 2  decreases. When the discharge valve  39  is closed, and the suction valve  38  is opened, the brake fluid as the working fluid flows from the suction-side space R 1  into the space R 2 , and the brake fluid is supplied from the suction oil passage  12  to the space R 2  via the suction port  823 . When the piston  36  performs a stroke away from the cam accommodating hole  81 , the volume of the space R 2  decreases, and the pressure in R 2  increases. When the suction valve  38  is closed, and the discharge valve  39  is opened, the brake fluid flows out from the space R 2  to the discharge-side space R 3 , and the brake fluid is supplied to the discharge oil passage  13  via the discharge port  821 . The brake fluid discharged by the respective pump parts  3 A to  3 E to the holes  88 - 31  to  88 - 38  is collected to the one hole  88 - 39  (discharge oil passage  13 ), and is used in common by the two systems of the hydraulic pressure circuit. The second unit  1 B is configured to supply the brake fluid pressurized by the pump  3  to the brake operation units via the wheel cylinder pipes  10 W, to thereby generate the brake hydraulic pressures (wheel cylinder pressures). The second unit  1 B can supply the master cylinder pressure to the respective wheel cylinders W/C, and can use the hydraulic pressure generated by the pump  3  to individually control the hydraulic pressures of the respective wheel cylinders W/C independently of the brake operation by the driver in the state in which the communication between the master cylinder  5  and the wheel cylinders W/C is closed. 
     (ECU Fixation) 
     An ECU  90  is arranged on, and mounted to the rear surface  802  of the housing  8 . In other words, the ECU  90  is integrally provided for the housing  8 . The ECU  90  includes a control board  900  and a control unit housing (case)  901 . The control board  900  is configured to control states of current supply to the motor  20  and the solenoids of the electromagnetic valves  21  and the like. Various sensors configured to detect a motion state of the vehicle, for example, an acceleration sensor configured to detect an acceleration of the vehicle and an angular velocity sensor configured to detect an angular velocity (yaw rate) of the vehicle may be mounted to the control board  900 . Moreover, a complex sensor (combined sensor) which is a unit of those sensors may be mounted to the control board  900 . The control board  900  is accommodated in the case  901 . The case  901  is a cover member fixed through fastening with bolts b 2  to the rear surface  802  (bolt holes  892 ) of the housing  8 . The rear surface  802  functions as a case mounting surface (cover member mounting surface). The bolt holes  892  function as fixing holes (fixing parts) for fixing the ECU  90  to the housing  8 . 
     The case  901  is a cover member made of a resin material, and includes a board accommodating part  902  and a connector part  903 . The board accommodating part  902  is configured to accommodate the control board  900  and some of the solenoids of the electromagnetic valves  21  and the like (hereinafter referred to as “control board  900  and the like”). The board accommodating part  902  includes a lid part  902   a . The lid part  902   a  is configured to cover the control board  900  and the like for isolation from the outside.  FIG. 16  is a diagram for illustrating the ECU  90  mounted to the housing  8  as viewed from the negative side in the Y-axis direction in the state in which the lid part  902   a  is removed. The control board  900  is mounted to the board accommodating part  902  approximately in parallel with the rear surface  802 . Terminals of the solenoids of the electromagnetic valves  21  and the like, terminals of the hydraulic pressure sensor  91  and the like, and the conductive members (not shown) from the motor  20  protrude from the rear surface  802 . The terminals and the conductive members extend to the negative side in the Y-axis direction, and are connected to the control board  900 . The connector part  903  is arranged on the negative side in the X-axis direction with respect to the terminals and the conductive members in the board accommodating part  902 , and protrudes toward a positive side in the Y-axis direction of the board accommodating part  902 . The connector part  903  is arranged slightly on the outside (on the negative side in the X-axis direction) with respect to the left side surface  806  of the housing  8  as viewed in the Y-axis direction. Terminals of the connector part  903  are exposed toward the positive side in the Y-axis direction, and extend to the negative side in the Y-axis direction so as to be connected to the control board  900 . Each of the terminals (exposed toward the positive side in the Y-axis direction) of the connector part  903  can be connected to external devices and the stroke sensor  94  (hereinafter referred to as “external devices and the like”). Electrical connections between the external devices and the like and the control board  900  (ECU  90 ) are achieved by another connector connected to the external devices and the like being inserted into the connector part  903  from the positive side in the Y-axis direction. Moreover, a current supply is carried out from an external power supply (battery) to the control board  900  via the connector part  903 . The conductive members function as a connection part configured to electrically connect the control board and (the stator of) the motor  20  to each other, and a current is supplied to (the stator of) the motor  20  from the control board  900  via the conductive members. 
     The ECU  90  is configured to receive input of detection values of the stroke sensor  94 , the hydraulic pressure sensor  91 , and the like, and information on the travel state from the vehicle side, and control the opening/closing operations of the electromagnetic valves  21  and the like and the number of revolutions (namely a discharge amount of the pump  3 ) of the motor  20  based on a built-in program, to thereby control the wheel cylinder pressures (hydraulic pressure braking forces) of the respective wheels FL to RR. With such control, the ECU  90  carries out various types of brake control (for example, antilock brake control for suppressing slip of wheels caused by the braking, boost control for decreasing a brake operation force of the driver, brake control for motion control for the vehicle, automatic brake control, for example, preceding vehicle following control, and regeneration cooperative brake control). The motion control for the vehicle includes stabilization control of vehicle behavior such as lateral slipping. The regeneration cooperative brake control controls the wheel cylinder hydraulic pressures so as to achieve a target deceleration (target braking forces) in cooperation with regenerative braking. 
     The ECU  90  includes a brake operation amount detection part  90   a , a target wheel cylinder hydraulic pressure calculation part  90   b , a stepping force braking generation part  90   c , a boost control part  90   d , and a control switching part  90   e . The brake operation amount detection part  90   a  is configured to receive input of the detection value of the stroke sensor  94 , to thereby detect a displacement amount (pedal stroke) of the brake pedal  100  as a brake operation amount. The target wheel cylinder hydraulic pressure calculation part  90   b  is configured to calculate target wheel cylinder hydraulic pressures. Specifically, the target wheel cylinder hydraulic pressure calculation part  90   b  is configured to calculate, based on the detected pedal stroke, the target wheel cylinder hydraulic pressures for achieving a predetermined boost ratio, namely an ideal relationship between the pedal stroke and the brake hydraulic pressures required by the driver (vehicle deceleration G required by the driver). Moreover, the target wheel cylinder hydraulic pressure calculation part  90   b  is configured to calculate the target wheel cylinder hydraulic pressures based on a relationship with a regenerative braking force during the regeneration cooperative brake control. For example, the target wheel cylinder hydraulic pressure calculation part  90   b  is configured to calculate such target wheel cylinder hydraulic pressures that a sum of a regenerative braking force input from a control unit of a regenerative braking device and a hydraulic pressure braking force corresponding to the target wheel cylinder hydraulic pressures satisfies the vehicle deceleration required by the driver. The target wheel cylinder hydraulic pressure calculation part  90   b  is configured to calculate the target wheel cylinder hydraulic pressures of the respective wheels FL to RR in order to achieve a desired vehicle motion state, for example, based on a detected vehicle motion state amount (for example, a lateral acceleration) during the motion control. 
     The stepping force braking generation part  90   c  is configured to set the pump  3  to a non-operation state, and control the shutoff valves  21  toward the open direction, control the SS/V IN  27  toward the closed direction, and control the SS/V OUT  28  toward the closed direction. In the state in which the shutoff valves  21  are controlled toward the open direction, the oil passage system (for example, the supply oil passages  11 ) connecting the hydraulic pressure chambers  50  of the master cylinder  5  and the wheel cylinders W/C to each other achieves stepping force braking (non-boost control) of generating the wheel cylinder hydraulic pressures through the master cylinder pressure generated by the pedal stepping force. The SS/V OUT  28  is controlled toward the closed direction, and the stroke simulator  6  does not thus function. In other words, the operation of the piston  61  of the stroke simulator  6  is suppressed, and the inflow of the brake fluid from the hydraulic pressure chamber  50  (secondary chamber  50 S) to the positive pressure chamber  601  is thus suppressed. As a result, the wheel cylinder hydraulic pressures can more efficiently be boosted. The S/V IN  27  may be controlled toward the closed direction. 
     In the state in which the SS/V IN  27  is controlled toward the closed direction, and the SS/V OUT  28  is controlled toward the open direction while the shutoff valves  21  are controlled toward the closed direction, a braking system (the suction oil passage  12 , the discharge oil passage  13 , and the like) connecting the reservoir  120  and the wheel cylinders W/C to each other functions as a so-called brake-by-wire system configured to generate the wheel cylinder hydraulic pressures through the hydraulic pressure generated by the pump  3 , to thereby achieve the boost control, the regeneration cooperative control, and the like. The boost control part  90   d  is configured to operate the pump  3 , control the shutoff valves  21  toward the closed direction, and control the communication valves  23  toward the open direction during the brake operation by the driver, to thereby bring the state of the second unit  1 B into a state in which the wheel cylinder hydraulic pressures can be generated by the pump  3 . As a result, the boost control part  90   d  is configured to carry out the boost control of using the discharge pressure of the pump  3  as a hydraulic pressure source to generate the wheel cylinder hydraulic pressures higher than the master cylinder pressure, to thereby generate the hydraulic pressure braking force that is not sufficiently generated by the brake operation force of the driver. Specifically, the boost control part  90   d  is configured to control the pressure regulating valve  24  while operating the pump  3  at a predetermined number of revolutions to adjust the brake fluid amount supplied from the pump  3  to the wheel cylinders W/C, to thereby achieve the target wheel cylinder hydraulic pressures. In other words, the braking system  1  is configured to operate the pump  3  of the second unit  1 B in place of an engine negative pressure booster, to thereby provide a boost function of assisting the brake operation force. Moreover, the boost control part  90   d  is configured to control the SS/V IN  27  toward the closed direction, and control the SS/V OUT  28  toward the open direction. With such control, the boost control part  90   d  causes the stroke simulator  6  to function. The control switching part  90   e  is configured to control the operation of the master cylinder  5 , to thereby switch between the stepping force braking and the boost control based on the calculated target wheel cylinder hydraulic pressures. Specifically, when the start of the brake operation is detected by the brake operation amount detection part  90   a , the control switching part  90   e  causes the stepping force braking generation part  90   c  to generate the wheel cylinder hydraulic pressures if the calculated target wheel cylinder hydraulic pressures are equal to or less than predetermined values (for example, values corresponding to the maximum value of the vehicle deceleration G generated during normal braking, which is not sudden braking). Meanwhile, if the target wheel cylinder hydraulic pressures calculated upon the brake stepping operation exceed the predetermined values, the control switching part  90   e  causes the boost control part  90   d  to generate the wheel cylinder hydraulic pressures. 
     Moreover, the ECU  90  includes a sudden brake operation state determination part  90   f  and a second stepping force braking generation part  90   g . The sudden brake operation state determination part  90   f  is configured to detect a brake operation state based on input from the brake operation amount detection part  90   a  and the like, to thereby determine whether or not the brake operation state is a predetermined sudden brake operation state. For example, the sudden brake operation state determination part  90   f  is configured to determine whether or not a change amount of the pedal stroke per unit time exceeds a predetermined threshold amount. The control switching part  90   e  is configured to switch the control so that the wheel cylinder hydraulic pressures are generated by the second stepping force braking generation part  90  when the brake operation state is determined to be the sudden brake operation state. The second stepping force braking generation part  90   g  is configured to operate the pump  3 , and to control the shutoff valves  21  toward the closed direction, control the SS/V IN  27  toward the open direction, and control the SS/V OUT  28  toward the closed direction. With such control, there is achieved second stepping force braking of using the brake fluid having flowed out from the back pressure chamber  602  of the stroke simulator  6  to generate the wheel cylinder hydraulic pressures until the pump  3  can generate sufficiently high wheel cylinder pressures. The shutoff valves  21  may be controlled toward the open direction. Moreover, the SS/V IN  27  may be controlled toward the closed direction, and, in this case, the brake fluid from the back pressure chamber  602  is supplied to the wheel cylinder W/C side via the check valve  270  (in the open state because the pressure on the wheel cylinder W/C side is still lower than that on the back pressure chamber  602  side). In this embodiment, the brake fluid can efficiently be supplied from the back pressure chamber  602  side to the wheel cylinder W/C side by controlling the SS/V IN  27  toward the open direction. Then, when the brake operation state is no longer determined to be the sudden brake operation state, and/or a predetermined condition indicating that a discharge performance of the pump  3  has become sufficient is satisfied, the control switching part  90   e  switches the control so as to cause the boost control part  90   d  to generate the wheel cylinder hydraulic pressures. In other words, the boost control part  90   d  controls the SS/V IN  27  toward the closed direction, and controls the SS/V OUT  28  toward the open direction. With such control, the boost control part  90   d  causes the stroke simulator  6  to function. The control may be switched to the regeneration cooperative brake control after the second stepping force braking. 
     A description is now given of the operation. 
     [Switching of Control] 
     The SS/V OUT  28 , the SS/V IN  27 , and the check valve  270  are configured to adjust the flow of the brake fluid, which has flowed out from the back pressure port  874  into the housing  8 . Those valves permit or inhibit the flow of the brake fluid, which has flowed from the back pressure port  874  into the housing  8 , to any of the low pressure parts (the reservoir  120  and the wheel cylinders W/C), to thereby permit or inhibit the flow of the brake fluid from the master cylinder  5  to the stroke simulator  6  (positive pressure chamber  601 ). With such actions, those valves adjust the operation of the stroke simulator  6 . Moreover, the SS/V OUT  28 , the SS/V IN  27 , and the check valve  270  function as a switching part configured to switch a supply destination (outflow destination) of the brake fluid, which has flowed from the back pressure port  874  into the housing  8  (back pressure oil passage  16 ), between the reservoir  120  and the wheel cylinders W/C. The control switching part  90   e  is configured to control the SS/V OUT  28  toward the closed direction so as to achieve the second stepping force braking until the pump  3  can come to be able to generate sufficiently high wheel cylinder pressures. As a result, the brake fluid, which has flowed from the back pressure chamber  602  of the stroke simulator  6  into the back pressure oil passage  16  via the back pressure pipe  10 X, flows toward the supply oil passages  11  via the SS/V IN  27  (fist simulator oil passage  17 ) and the check valve  270  (bypass oil passage  170 ). In other words, the supply destination of the brake fluid flowing from the back pressure chamber  602  is switched to the wheel cylinders W/C. Thus, boost responsiveness of the wheel cylinder hydraulic pressures can be secured. When the pressure on the wheel cylinder W/C side exceeds the pressure on the back pressure chamber  602  side, the check valve  270  is automatically closed, and a counter flow of the brake fluid from the wheel cylinder W/C side to the back pressure chamber  602  side is suppressed. When the brake operation state is determined to be the sudden brake operation state, the control switching part  90   e  controls the SS/V OUT  28  toward the closed direction, to thereby switch the supply destination of the brake fluid to the wheel cylinders. Thus, the second stepping force braking can appropriately be achieved when the boost responsiveness of the wheel cylinder hydraulic pressures is required. The pump  3  is not limited to the piston pump, and may be, for example, a gear pump. According to this embodiment, the pump  3  is the piston pump, and thus the responsiveness is relatively high. Thus, a period until the pump  3  comes to be able to generate sufficient wheel cylinder pressures after start of the operation is relatively short, and a period in which the second stepping force braking is operating can thus be decreased. When the predetermined condition indicating that the discharge performance of the pump  3  has become sufficient is satisfied, the control switching part  90   e  controls the SS/V OUT  28  toward the open direction in order to cause the stroke simulator  6  to function. As a result, the brake fluid, which has flowed from the back pressure chamber  602  of the stroke simulator  6  into the back pressure oil passage  16  via the back pressure pipe  10 X, flows toward the reservoir  120  via the SS/V OUT  28  (second simulator oil passage  18 ). In other words, the supply destination of the brake fluid flowing from the back pressure chamber  602  is the reservoir  120 . Thus, excellent pedal feeling can be secured. Even when such a failure that the SS/V OUT  28  is stuck in the closed state occurs during operation of the stroke simulator  6 , the piston  61  can return to the initial position by the brake fluid being supplied from the reservoir  120  side to the back pressure chamber  602  via the check valve  280 . 
     [Distribution of Respective Members to First and Second Units] 
     The braking system  1  includes the first unit  1 A and the second unit  1 B. Mountability of the braking system  1  to the vehicle can thus be improved. The stroke simulator  6  is arranged in the first unit  1 A. Thus, compared with a case in which the stroke simulator  6  is separate from the master cylinder  5  or the second unit  1 B, the lengths of pipes that connect the master cylinder  5  or the second unit  1 B and the stroke simulator  6  to each other can be decreased, and the number of the pipes can be decreased. Thus, an increase in complexity of the braking system  1  can be suppressed, and an increase in cost caused by the increase in the number of pipes can be suppressed. The stroke simulator  6  may be arranged in the second unit  1 B. In this embodiment, the stroke simulator  6  is arranged in the first unit  1 A, and the master cylinder  5  and the stroke simulator  6  are integrated into the first unit  1 A. Thus, an increase in size of the second unit  1 B can be suppressed compared with the case in which the stroke simulator  6  is arranged in the second unit  1 B. A housing of the master cylinder  5  and a housing of the stroke simulator  6  may be provided independently of each other, and may be arranged, for example, spatially closely but separately. In this embodiment, the housing  7  of the master cylinder  5  and the housing  7  of the stroke simulator  6  are integrally provided. Thus, a pipe that connects the master cylinder  5  and the stroke simulator  6  to each other can be omitted. Specifically, the positive pressure oil passage  74  that connects the secondary chamber  50 S of the master cylinder  5  and the positive pressure chamber  601  of the stroke simulator  6  to each other is formed inside the housing  7 . Thus, the pipe that connects the secondary chamber  50 S and the positive pressure chamber  601  to each other can be omitted. The housing of the master cylinder  5  and the housing of the stroke simulator  6  may be provided independently of each other, and may integrally be fixed to each other. In this embodiment, the housing  7  of the master cylinder  5  and the housing  7  of the stroke simulator  6  are shared in common. Thus, the positive pressure oil passage  74  can easily be formed inside the housing  7 . The pipe that connects the stroke simulator  6  and the second unit  1 B to each other does not include a pipe that connects the positive pressure chamber  601  and the second unit  1 B to each other, and includes only the back pressure pipe  10 X that connects the back pressure chamber  602  and the second unit  1 B to each other. Thus, the number of the pipes that connect the first unit  1 A (stroke simulator  6 ) and the second unit  1 B to each other can be decreased. Moreover, the back pressure pipe  10 X extending from the back pressure chamber  602  is connected to the second unit  1 B. Thus, a pipe or an oil passage that connects the back pressure chamber  602  (stroke simulator  6 ) and the reservoir tank  4  to each other is not necessary in the first unit  1 A, and the size of the first unit  1 A can be decreased. 
     The electromagnetic valves, the hydraulic pressure sensor  91 , and the like are arranged in the second unit  1 B. Thus, an ECU for driving the electromagnetic valves is not required in the first unit  1 A, and wires (harness) for the electromagnetic valve control and sensor signal transmission are not necessary between the first unit  1 A and the ECU  90  (second unit  1 B). Thus, an increase in complexity of the braking system  1  can be suppressed, and an increase in cost caused by an increase in the number of wires can be suppressed. Moreover, the ECU is not arranged in the first unit  1 A, and thus the size of the first unit  1 A can be decreased, and the degree of freedom in layout can be increased. For example, the SS/V IN  27  and the SS/V OUT  28  are arranged in the second unit  1 B. Thus, the first unit  1 A does not need an ECU for switching the operation of the stroke simulator  6 , and wires (harness) for controlling the SS/V IN  27  and the SS/V OUT  28  are not necessary between the first unit  1 A and the ECU  90  (second unit  1 B). The ECU  90  is arranged in the second unit  1 B, and the ECU  90  and the housing  8  (that accommodates the electromagnetic valves and the like) are integrated with each other as the second unit  1 B. Thus, wires (harness) that connect the electromagnetic valves, the hydraulic pressure sensor  91 , and the like and the ECU  90  to each other can be omitted. Specifically, terminals of solenoids of the electromagnetic valves  21  and the like and terminals of the hydraulic pressure sensor  91  and the like are directly connected to the control board  900  (without via harnesses and connectors outside the housing  8 ). For example, the harness that connects the ECU  90 , and the SS/V IN  27  and the SS/V OUT  28  to each other can be omitted. The motor  20  is arranged in the second unit  1 B, and the housing  8  (that accommodates the pump  3 ) and the motor  20  are integrated with each other as the second unit  1 B. The second unit  1 B functions as the pump device. Thus, wires (harness) that connect the motor  20  and the ECU  90  to each other can be omitted. Specifically, the conductive members for the current supply and the signal transmission to the motor  20  are accommodated in the power supply hole  86  of the housing  8 , and are directly connected (without via harnesses and connectors outside the housing  8 ) to the control board  900 . The conductive members function as connection members that connect the control board  900  and the motor  20  to each other. 
     [About First Unit  1 A] 
     The reservoir tank  4  is arranged in the uppermost part in the vertical direction of the first unit  1 A in a state in which the first unit  1 A is mounted to the vehicle. Thus, supplement of the brake fluid to the reservoir tank  4  and inspection of the amount of brake fluid can easily be performed. The stroke simulator  6  overlaps with the master cylinder  5  as viewed in the vertical direction. A projection area of the first unit  1 A in the vertical direction can thus be decreased, thereby being capable of improving the mountability of the first unit  1 A to the vehicle. An axial center direction of the piston  51  of the master cylinder  5  is approximately orthogonal to the vertical direction. An axial center direction of the piston  61  of the stroke simulator  6  approximately matches the axial center direction of the piston  51 . Thus, an area in which the stroke simulator  6  and the master cylinder  5  overlap with each other as viewed in the vertical direction can be increased, and the projection area in the vertical direction of the first unit  1 A can be decreased. The reservoir tank  4  overlaps with the master cylinder  5  and the stroke simulator  6  as viewed in the vertical direction. Thus, the projection area of the first unit  1 A in the vertical direction can be decreased. In this embodiment, most of the master cylinder  5  and the stroke simulator  6  are covered by the reservoir tank  4  as viewed in the vertical direction. It is preferred that portions constructing the ports  76  and  77  for the pipe connection be not covered by the tank  4 , and be thus exposed as viewed in the vertical direction. In this case, connection workability of the pipes  10 M and  10 X to the ports  76  and  77  can be improved. The reservoir tank  4 , the master cylinder  5 , and the stroke simulator  6  are within the width of the flange part  78  in the Y-axis direction. Thus, a size of the first unit  1 A can be decreased in the lateral direction of the vehicle orthogonal to the pushrod  101 . Therefore, the mountability of the first unit  1 A to the vehicle can be improved. 
     [About Second Unit  1 B] 
     (Pump Pulse Pressure Reduction) 
     The pump  3  may include a piston that is reciprocated by the motion of the cam, and a specific configuration is not limited to that of this embodiment. For example, the number of the pump parts (pistons  36 ) may be one or two, and is not limited to five. In this embodiment, the plurality of pump parts are provided. Thus, a phase of suction/discharge strokes of the respective pump parts  3 A to  3 E can be displaced from one another. As a result, periodical variations (pulse pressures) of discharge pressure of the respective pump parts  3 A to  3 E can be canceled one another, and the pulse pressure in the entire pump  3  can be reduced. In other words, the pulsation of the flow in the hole  88 - 39  (discharge oil passage  13 ) into which the respective pump parts  3 A to  3 E discharge in common the brake fluid can be suppressed to be low, thereby being capable of decreasing noise and vibration of the braking system  1 . 
     The respective pistons  36  are arranged at approximately equal intervals in the circumferential direction. In other words, the respective pistons  36  are arranged approximately equiangularly in the circumferential direction. Thus, phase displacements of the suction/discharge strokes can be approximately even between the pump parts  3 A to  3 E, thereby being capable of attaining a significant pulse pressure reduction effect.  FIG. 17  to  FIG. 21  are graphs for showing results of verification of a relationship between the rotation angle θ of the rotation shaft of the motor  20  (pump rotation shaft  300 ) and a load torque F acting on the rotation shaft of the motor  20  (pump rotation shaft  300 ) for the pumps  3  that include a plurality of the pump parts having the same size and other configurations, and in which the respective pistons  36  are arranged at approximately equal intervals in the circumferential direction.  FIG. 17  is a graph for showing a first example in which the number of pump parts (pistons  36 ) is two.  FIG. 18  is a graph for showing a second example in which the number is three.  FIG. 19  is a graph for showing a third example in which the number of pump parts is four.  FIG. 20  is a graph for showing a fourth example in which the number of pump parts is five.  FIG. 21  is a graph for showing a fifth example in which the number of pump parts is six. The load torque generated in each pump part  3   n  is indicated as Fn. The suffix “n” is provided for discrimination of the respective pump parts from one another, and represents a natural number from 2 to 6. Fn approximately corresponds to a force which is generated by the discharge pressure and acts on the piston  36   n  of the pump part  3   n . In a half cycle in which the pump part  3   n  is in the discharge stroke, the force (pressure on the discharge side in the passage) caused by the discharge pressure changes in a sine waveform in accordance with the stroke (volume change in the space R 2 ) of the piston  36   n  caused by the change in θ, and Fn thus changes in a sine waveform while 0 is reference, with respect to the change in θ. In a half cycle in which the pump part  3   n  is in the suction stroke, the force caused by the discharge pressure can be considered as 0, and Fn thus remains as 0 with respect to the change in θ. The load torque F in the entire pump  3  is a sum of the Fns for all ns for each θ. The pulse pressure (specifically, a magnitude thereof) in the entire pump  3  corresponds to a variation (width) of the F as a whole. The respective pistons  36  are arranged at approximately equal intervals in the circumferential direction, and thus the respective Fns change while the phases are displaced approximately by 360/n (°) from each other. Thus, the variation width ΔF of the F as a whole acquired as the sum of the respective Fns decreases. 
     The number of the pump parts  3  is not limited to five, and may be an even number. The pulse pressure reduction effect corresponding to the number of pump parts can be verified by observing the variation width ΔF. Table 1 shows ΔF, the number of peaks of F per one revolution of the pump rotation shaft  300 , and a ratio of ΔF to the amplitude F 0  of Fn (hereinafter referred to as “amplitude ratio”) for the respective pumps  3  (respective numbers of the pump parts) of  FIG. 17  to  FIG. 21 . 
                             TABLE 1               Number of pump parts   Number of peaks   Amplitude ratio       (number)   (number/rev)   (%)                                            2   2   100       3   6   14       4   4   41       5   10   6       6   6   27                    
In the first example in which the number of the pump parts is two, the number of peaks of F is two, and the amplitude of Fn and ΔF are the same (amplitude ratio is 100%). In the third example in which the number of the pump parts is four, the number of peaks of F is four, and the amplitude ratio is 41%. In the fifth example in which the number of the pump parts is six, the number of peaks of F is six, and the amplitude ratio is 27%. When the number of the pump parts is an even number, the number of peaks of F is equal to the number of the pump parts in this way. Moreover, as the number of the pump parts increases, the amplitude ratio decreases. Meanwhile, in the second example in which the number of the pump parts is three, the number of peaks of F is six, and the amplitude ratio is 14%. In the fourth example in which the number of the pump parts is five, the number of peaks of F is ten, and the amplitude ratio is 6%. When the number of the pump parts is an odd number, the number of peaks of F is equal to the twice of the number of the pump parts in this way. Moreover, as the number of the pump parts increases, the amplitude ratio decreases. When the number of the pump parts is an odd number, the number of peaks of F increases, and the amplitude ratio significantly decreases compared with a case in which the number of the pump parts is an even number. In other words, it is understood that in the entire pump  3 , the discharge pressure is smoothed and the variation (pulse pressure) is reduced.
 
     In this embodiment, the number of the pump parts is an odd number equal to or more than three. Thus, the amplitude of the pulse pressure can easily be decreased compared with the cases in which the number of the pump parts is an even number, and the significant pulse pressure reduction effect can be attained. For example, when the number of the pump parts is three, there can be attained the pulse pressure reduction effect greater than that of the case in which the number is six. In this embodiment, the number of the pump parts is five. Thus, the pulse pressure reduction effect can be improved, thereby being capable of attaining sufficient silence, and securing a sufficient flow rate of the pump  3  compared with the case in which the number is three. Moreover, compared with the case in which the number is six or more, the increase in the number of the pump parts  3  can be suppressed, which is advantageous in terms of the layout and the like, and the size of the second unit  1 B can easily be decreased. The brake fluid in the hole  88 - 39  flows to the hole  88 - 310  via the dumper chamber  831 . A radial sectional area of the damper chamber  831  is larger than flow passage cross sectional areas of the respective holes  88 - 39  and  88 - 310 . In other words, the damper chamber  831  is a volume chamber in the oil passages. The damper chamber  831  functions as the damper  130 , and is configured to absorb pulsation of the brake fluid in the discharge oil passage  13  discharged from the pump  3 . As a result, the pulse pi ensure is further reduced. 
     (Improvement in Workability) 
     The master cylinder ports  871  and the wheel cylinder ports  872  are arranged on the upper side in the vertical direction of the housing  8 . Thus, workability of respectively mounting the pipes  10 MP,  10 MS, and  10 W to the ports  871  and  872  of the housing  8  provided on the vehicle body side can be improved. The wheel cylinder ports  872  are opened in the top surface  803 . Therefore, the workability can further be improved. The master cylinder ports  871  are opened at the end on the upper side in the vertical direction of the front surface  801 . Therefore, the workability can further be improved. 
     (Reservoir Function) 
     The reservoir chamber  830  is configured to receive the brake fluid supplemented from the reservoir tank  4  via the pipe  10 R, and supply the brake fluid to the suction ports  823  of the respective pump parts  3 A to  3 E. The respective pump parts  3 A to  3 E are configured to suck and discharge the brake fluid via the reservoir  120 . The reservoir chamber  830  is a volume chamber in the oil passages. When the suction pipe  10 R is detached from the nipple  10 R 1  or  10 R 2 , or when a band for tightening the suction pipe  10 R to the nipple  10 R 1  or  10 R 2  is loosened, and the brake fluid thus leaks from the suction pipe  10 R, the reservoir chamber  830  functions as the reservoir  120  that is configured to reserve the brake fluid. The pump  3  can suck and discharge the brake fluid in the reservoir  120 , to thereby generate the wheel cylinder pressures, and can generate the braking torque in the vehicle in which the braking system  1  is mounted. The suction port  873  is formed on the upper side in the vertical direction with respect to the intake ports  823  of the pump parts  3 A to  3 E. Thus, even when leakage of a fluid from the suction pipe  10 R occurs, the brake fluid can be reserved in at least some of oil passages extending from the suction port  873  to the suction ports  823  of the pump  3 , and the pump  3  can use this brake fluid to generate the discharge pressure. In other words, at least some of the oil passages in which the brake fluid is reserved can be caused to function as the reservoir  120 . It is not always required that the suction port  873  be opened in the top surface  803 . The suction port  873  in this embodiment is opened in the top surface  803 . In other words, the suction port  873  is formed toward the top side in the vertical direction, and is opened in the top side in the vertical direction. Thus, the brake fluid can be reserved in entire oil passages extending from the suction port  873  to the suction ports  823  of the pump  3 . It is preferred that the suction port  873  be positioned on a lower side in the vertical direction with respect to the supply port  41  of the reservoir tank  4 . In this case, the brake fluid can always be supplemented from the reservoir tank  4  to the suction port  873  via the pipe  10 R. 
     It is preferred that the reservoir chamber  830  has a capacity (volume) enabling the vehicle in which the braking system  1  is mounted to use the pump  3  to generate a predetermined braking torque (for example, −0.25 G). In this case, even when the liquid leak from the suction pipe  10 R occurs, the brake control by the pump  3  can be continued by using the brake fluid in the reservoir  120 . The reservoir chamber  830  is arranged on the upper side in the vertical direction with respect to the intake ports  823  of the pump parts  3 A to  3 E. Thus, the brake fluid can easily be supplied from the reservoir chamber  830  to the suction ports  823  of the pump  3 . The suction port  873  may be connected to the reservoir chamber  830  via an oil passage. In this embodiment, the suction port  873  is directly connected to the reservoir chamber  830 . In other words, the reservoir chamber  830  is opened in the top surface  803 , and this opening functions as the suction port  873 . The reservoir chamber  830  includes the suction port  873 , and is opened in the suction port  873 . Thus, the one end of the reservoir chamber  830  can be arranged as close to the top surface  803  side as possible, and a large substantial capacity of the reservoir  120  can be secured. Moreover, the reservoir chamber  830  is opened in the upper side in the vertical direction. Thus, even when the liquid leak from the suction pipe  10 R occurs, leakage of the brake fluid from the reservoir chamber  830  is suppressed. Thus, the reservoir chamber  830  can be caused to function as the reservoir  120 . 
     (Drain Function) 
     The brake fluid leaks from each of the cylinder accommodating holes  82  to the cam accommodating hole  81  via the first seal ring  351 . For example, the brake fluid leaks from the suction-side space R 1  via a gap between the piston  36  and the first seal ring  351 . The brake fluid that has leaked into the cam accommodating hole  81  flows into the liquid reservoir chamber  832  via the oil passage hole  881 , and is reserved in the liquid reservoir chamber  832 . Thus, entry of the brake fluid in the cam accommodating hole  81  into the motor  20  is suppressed, and an operation performance of the motor  20  can be improved. The liquid reservoir chamber  832  is arranged on the negative side in the Z-axis direction with respect to the cam accommodating hole  81 . Thus, the brake fluid that has leaked from each of the cylinder accommodating holes  82  into the cam accommodating hole  81  can flow by its own weight from the cam accommodating hole  81  to the liquid reservoir chamber  832 . As a result, the leaked brake fluid can efficiently be reserved in the liquid reservoir chamber  832 . The liquid reservoir chamber  832  is opened in the bottom surface  804 . Thus, the one end of the liquid reservoir chamber  832  can be arranged as close to the bottom surface  804  side as possible, and a large substantial capacity of the liquid reservoir chamber  832  can be secured. The opening of the liquid reservoir chamber  832  is closed by a lid member. Moreover, an amount of the brake fluid exceeding the capacity of the liquid reservoir chamber  832  can be returned to the suction ports  823  of the pump  3  via the hole  88 - 46 . 
     (Suppression of Air Stagnation) 
     When the housing  8  is viewed along the vertical direction, the holes which are subject to high pressure are mainly formed on the lower side in the vertical direction with respect to the axial center O, and the holes which are subject to low pressure are mainly formed on the upper side in the vertical direction. Thus, stagnation of the air in the oil passages connecting those holes can be suppressed. For example, the damper chamber  831  is arranged on the lower side in the vertical direction with respect to the cam accommodating hole  81 . Thus, the brake fluid at high pressure discharged from the discharge ports  821  of the pump  3  into the damper chamber  831  can be caused to flow from the lower side in the vertical direction of the housing  8  to the upper side in the vertical direction. The damper chamber  831  is opened in the bottom surface  804 . Thus, the damper chamber  831  can be arranged as close to the bottom side in the vertical direction as possible, and a dead space on the lower side in the vertical direction with respect to the damper chamber  831  can be decreased in the housing  8 . In other words, the holes which are subject to relatively high pressure and are on an upstream side of the flow of the brake fluid are arranged on the lower side in the vertical direction of the housing  8 , and the holes which are subject to relatively low pressure and are on a downstream side of the flow of the brake fluid are arranged on the upper side in the vertical direction of the housing  8 . As a result, the flow of the brake fluid tends to be directed from the lower side in the vertical direction of the housing  8  to the upper side in the vertical direction. Thus, stagnation of air (air bubbles) in the oil passages can be suppressed. For example, the communication valve accommodating holes  843  and the pressure regulating valve accommodating hole  844  immediately communicating with the damper chamber  831  are subject to high pressure, and are thus arranged on the lower side in the vertical direction of the housing  8 . The SOL/V IN accommodating holes  842  and the SOL/V OUT accommodating holes  845  are on a downstream side of the communication valve accommodating holes  843  and the pressure regulating valve accommodating hole  844 , and are thus arranged on the upper side in the vertical direction of the housing  8 . When the SS/V IN  27  is opened, the SS/V IN accommodating hole  847  is on an upstream side with respect to the shutoff valve accommodating holes  841 , and the SS/V IN accommodating hole  847  is thus arranged on the lower side in the vertical direction with respect to the shutoff valve accommodating hole  841 , specifically, on the lower side in the vertical direction with respect to the axial center O. 
     (Decrease in Size and Improvement in Ease of Layout) 
     The housing  8  is arranged between the motor  20  and the ECU  90 . Specifically, the motor  20 , the housing  8 , and the ECU  90  are arrayed in this order along the axial center direction of the motor  20 . Thus, the motor  20  and the ECU  90  can be arranged so as to overlap with each other as viewed from the motor  20  side (in the axial center direction of the motor  20 ) or the side of the ECU  90 . As a result, the area of the second unit  1 B as viewed from the motor  20  side or the ECU  90  side can be decreased, and the size of the second unit  1 B can thus be decreased. The weight of the second unit  1 B can be decreased by decreasing the size of the second unit  1 B. 
     The connector part  903  of the ECU  90  is adjacent to (the left side surface  806  of) the housing  8  as viewed from the motor  20  side (in the axial center direction of the motor  20 ). In other words, the connector part  903  is not covered by the housing  8 , and protrudes from the side surface  806  of the housing  8  as viewed from the motor  20  side. Thus, an increase in dimension of the second unit  1 B in the direction (Y-axis direction) along the axial center of the motor  20  can be suppressed. The terminals of the connector part  903  are exposed toward the motor  20  side (positive side in the Y-axis direction). Thus, a connector (harness) connected to the connector part  903  overlaps with the housing  8  and the like in the axial center direction (Y-axis direction) of the motor  20 , and an increase in dimension in the Y-axis direction (axial center direction of the motor  20 ) of the second unit  1 B including the connector (harness) can be suppressed. The connector part  903  is adjacent to the left side surface  806  of the housing  8 . Thus, compared with a case in which the connector part  903  is adjacent to the top surface  803  of the housing  8 , interference between the connector (harness) connected to the connector part  903  and the pipes  10 MP and  10 MS connected to the master cylinder ports  871  can be suppressed. Moreover, interference between the vehicle-body-side member (mount  102 ) to which the bottom surface  804  is opposed, and the connector (harness) can be suppressed compared with a case in which the connector part  903  is adjacent to the bottom surface  804  of the housing  8 . The connector part  903  may be adjacent to the right side surface  805  of the housing  8 . In this embodiment, the connector part  903  is adjacent to the left side surface  806  of the housing  8 . Ports, for example, the back pressure port  874 , are not formed on the left side surface  806 . Thus, compared with a case in which the connector part  903  is adjacent to the right side surface  805  of the housing  8 , interference between the connector (harness) connected to the connector part  903  and the pipe  10 X connected to the back pressure port  874  can be suppressed. In other words, when the connector (harness) is connected to the connector part  903 , the connection can easily be carried out. Thus, mounting workability of the braking system  1  in the vehicle can be increased. 
     The housing  8  includes the plurality of cylinder accommodating holes  82  configured to accommodate the pistons  36  of the pump  3  and the plurality of the valve body accommodating holes  84  configured to accommodate the valve bodies of the electromagnetic valves  21  and the like. Those cylinder accommodating holes  82  and the valve body accommodating holes  84  at least partially overlap with each other as viewed from the motor  20  side (in the axial center direction of the motor  20 ). Thus, the area of the second unit  1 B as viewed from the motor  20  side (in the axial center direction of the motor  20 ) can be decreased. The plurality of the cylinder accommodating holes  82  are provided in the radiation form about the axial center O of the motor  20 . Thus, there can be provided a region in which the respective cylinder accommodating holes  82 A to  82 E overlap with one another in the axial center direction of the motor  20 . As a result, an increase in dimension of the housing  8  in the axial center direction of the motor  20  can be suppressed. As viewed from the motor  20  side (in the axial center direction of the motor  20 ), most of the plurality of the valve body accommodating holes  84  are contained in the circle connecting the ends of the cylinder accommodating holes  82  on the large-diameter part  821  side (side farther from the axial center O) to each other. In addition, the outer periphery of this circle and the valve body accommodating holes  84  can also at least partially overlap with each other. Thus, the area of the second unit  1 B as viewed from the motor  20  side (in the axial center direction of the motor  20 ) can be decreased. The number of the plurality of cylinder accommodating holes  82  is five. Thus, a distance between the cylinder accommodating holes  82  which are adjacent to each other is short in the circumferential direction about the axial center O. However, the cylinder accommodating holes  82  and the valve body accommodating holes  84  at least partially overlap with each other as viewed from the motor  20  side (in the axial center direction of the motor  20 ), and most of the plurality of the valve body accommodating holes  84  can thus be contained in the above-mentioned circle. 
     The two cylinder accommodating holes  82 A and  82 E on the positive side in the Z-axis direction are arranged on both the sides in the X-axis direction with respect to the axial center O. Thus, the cylinder accommodating hole  82  is not opened at the center in the X-axis direction close to the axial center O on the top surface  803 , and a large space can be secured for opening the other hole (reservoir chamber  830 ). The cylinder accommodating holes  82 A to  82 E are arrayed in the single row along the axial center direction of the motor  20 . Specifically, the axial centers  360  of the cylinder accommodating holes  82 A to  82 E are approximately on the same plane a that is approximately orthogonal to the axial center O. Thus, the cam unit  30  can be used in common for the plurality of pistons  36 , an increase in the number of the cam units  30  can thus be suppressed, and an increase in the number of the components and cost can be suppressed. Moreover, the pump rotation shaft  300  can be shortened by suppressing the increase in the number of the cam units  30 , and an increase in dimension of the housing  8  in the axial center direction of the motor  20  can thus be suppressed. As a result, the size and the weight of the second unit  1 B can be decreased. Moreover, the increase in dimension of the housing  8  in the axial center direction of the motor  20  can effectively be suppressed by maximizing a region of the overlap between the respective cylinder accommodating holes  82 A to  82 E in the Y-axis direction. The cylinder accommodating holes  82  are arranged on the front surface  801  side (on the side on which the motor  20  is mounted) of the housing  8 . Thus, the pump rotation shaft  300  can be further shortened. 
     The recessed parts  807  and  808  are formed at the corners on the front surface  801  side and the top surface  803  side of the housing  8 . Thus, the volume and the weight of the housing  8  can be decreased. The cylinder accommodating holes  82 A and  82 E are opened in the recessed parts  807  and  808 . Thus, an increase in dimension in the axial center direction of the cylinder accommodating holes  82 A and  82 E can be suppressed, thereby being capable of improving ease of assembly of the pump components to those holes  82 A and  82 E. 
     The plurality of valve body accommodating holes  84  are arrayed in the single row along the axial center direction of the motor  20 . As a result, the increase in dimension of the housing  8  in the axial center direction of the motor  20  can be suppressed. The valve body accommodating holes  84  are arranged on the rear surface  802  side (side on which the ECU  90  is mounted) of the housing  8 . Thus, electrical connectivity between the ECU  90  and solenoids of the electromagnetic valves  21  and the like can be improved. Specifically, the axial centers of the plurality of valve body accommodating holes  84  are approximately in parallel with the axial center of the motor  20 , and all of the valve body accommodating holes  84  are opened in the rear surface  802 . Thus, the solenoids of the electromagnetic valves  21  and the like can be arranged in a concentrated manner on the rear surface  802  of the housing  8 , thereby being capable of simplifying electrical connections between the ECU  90  and the solenoids. Similarly, the plurality of sensor accommodating holes  85  are arranged on the rear surface  802  side. Thus, the electrical connectivity between the ECU  90  and the hydraulic pressure sensors  91  and the like can be improved. The control board  900  of the ECU  90  is arranged approximately in parallel with the rear surface  802 . Thus, the electrical connection between the ECU  90  and the solenoids (and the sensors) can be simplified. 
       FIG. 22  is a right side view for illustrating the second unit  1 B as viewed from the positive side in the X-axis direction, and is an illustration of the passages and the like with transparency in the housing  8 . Illustration of components, for example, the pump  3  and the electromagnetic valves  21  is omitted. The housing  8  includes a pump region (pump part) β and an electromagnetic valve region (electromagnetic valve part) γ arranged in this order from the front surface  801  side toward the rear surface  802  side along the axial center direction of the motor  20 . A region in which the cylinder accommodating holes  82  are located is the pump region β, and a region in which the valve body accommodating holes  84  are located is the electromagnetic valve region γ, along the axial center direction of the motor  20 . The increase in dimension of the housing  8  in the axial center direction of the motor  20  is easily suppressed by arranging the cylinder accommodating holes  82  and the valve body accommodating holes  84  in the respective regions in the axial center direction of the motor  20  in a concentrated manner. Moreover, ease of layout of the respective elements in the housing  8  can be increased, and the size of the housing  8  can be decreased. In other words, the degree of freedom in layout of the plurality of holes on a plane orthogonal to the axial center of the motor  20  is improved in each of the regions β and γ. For example, the plurality of valve body accommodating holes  84  can easily be arranged so as to suppress an increase in dimension of the housing  8  on the plane in the electromagnetic valve region γ. Both the regions β and γ may partially overlap with each other in the axial center direction of the motor  20 . 
     Approximately the same numbers of the plurality of valve body accommodating holes  84  are respectively formed on the both sides in the Z-axis direction with respect to the axial center O. Specifically, the number of the valve accommodating holes  84  is 15, slightly more than eight thereof are formed on the positive side in the Z-axis direction with respect to the axial center O, and a slightly less than seven thereof are formed on the negative side in the Z-axis direction. Therefore, concentration of the valve body accommodating holes  84  on one side of the axial center O in the Z-axis direction and a consequent unbalanced increase in dimension of the housing  8  can be suppressed. Approximately the same numbers of the plurality of valve body accommodating holes  84  are respectively formed on the both sides in the X-axis direction with respect to the axial center O. Thus, concentration of the valve body accommodating holes  84  on one side of the axial center O in the X-axis direction and a consequent unbalanced increase in dimension of the housing  8  can be suppressed. Specifically, the holes  84  and  85  in the P system are mainly arranged on the positive side in the X-axis direction with respect to the axial center O, and the holes  84  and  85  in the S system are mainly arranged on the negative side in the X-axis direction. Thus, approximately the same numbers of the holes  84  and  85  can easily be formed on both sides in the X-axis direction with respect to the axial center O. 
     The plurality of valve body accommodating holes  84  are arranged in two rows in the Z-axis direction on the positive side in the Z-axis direction with respect to the axial center O, and in three rows in the Z-axis direction on the negative side in the Z-axis direction with respect to the axial center O. The three rows on the negative side in the Z-axis direction partially overlap with each other in the Z-axis direction. Thus, even on the negative side in the Z-axis direction, the dimension in the Z-axis direction substantially corresponds to approximately two rows. Thus, the dimensions in the Z-axis direction of the housing  8  can approximately be the same on the both sides in the Z-axis direction with respect to the axial center O. Specifically, in the P system, the opening of the pressure regulating valve accommodating hole  844  and the opening of the communication valve accommodating hole  843 P, and the opening of the shutoff valve accommodating hole  841 P and the opening of the SS/V IN accommodating hole  847  partially overlap with each other in the Z-axis direction (as viewed in the X-axis direction). The same holds true for the S system. Thus, an increase in dimension in the Z-axis direction of the rear surface  802  can be suppressed. 
     The plurality of valve body accommodating holes  84  are in four rows in the X-axis direction on the positive side in the Z-axis direction with respect to the axial center O. Thus, the electromagnetic valves (SS/V IN  22  and the like) can easily be arranged so as to correspond to the four wheels FL to RR. The plurality of valve body accommodating holes  84  are formed in five rows in the X-axis direction on the negative side in the Z-axis direction with respect to the axial center O, and partially overlap with one another in the X-axis direction. Thus, even on the negative side in the Z-axis direction, the dimension in the Z-axis direction substantially corresponds to approximately four rows. Thus, the dimensions in the X-axis direction can approximately be the same on the both sides in the Z-axis direction with respect to the axial center of the motor  20 . Specifically, in the P system, the opening of the pressure regulating valve accommodating hole  844  and the opening of the shutoff valve accommodating hole  841 P partially overlap with each other in the X-axis direction (as viewed in the Z-axis direction), and the opening of the communication valve accommodating hole  843 P and the opening of the SS/V IN accommodating hole  847  partially overlap with each other in the X-axis direction (as viewed in the Z-axis direction). The same holds true for the S system. Thus, an increase in dimension in the X-axis direction of the rear surface  802  can be suppressed. 
     On the negative side in the Z-axis direction with respect to the axial center O, the plurality of valve body accommodating holes  84  are formed in a staggered pattern (so as to alternate), and the openings of the valve accommodating holes  84  partially overlap with one another in the X-axis direction and the Z-axis direction on the rear surface  802 . Thus, as described above, the pressure regulating valve accommodating hole  844  can be formed at an intermediate position between the groups of the valve body accommodating holes  84  in both the P and S systems while the increases in dimension in the Z-axis direction and the X-axis direction are suppressed on the rear surface  802 . As a result, when the one pressure regulating valve is used both in the P and S systems, the pressure regulating valve accommodating hole  844  can easily be connected to the oil passages in both the systems, thereby simplifying the oil passage configuration. Moreover, the space can effectively be used by forming the sensor accommodating holes  85  between the plurality of valve body accommodating holes  84 . 
     The plurality of valve body accommodating holes  84  are formed so that valves having the same function or valves functionally close to one another in the distance in the hydraulic pressure circuit are arranged in the rows as viewed in the X-axis direction. Thus, the layout of the oil passages in the housing  8  can be simplified, thereby being capable of suppressing an increase in size of the housing  8 . The respective SOL/V INs  22  have the same function, and are thus arranged in a row in the X-axis direction. The respective SOL/V OUTs  25  have the same function, and are thus arranged in a row in the X-axis direction. The communication valves  23  and the pressure regulating valve  24  are functionally close to each other in the distance in the hydraulic pressure circuit, and are thus arranged in a row in the X-axis direction. The SS/V IN  27  and the SS/V OUT  28  are functionally close to each other in the distance in the hydraulic pressure circuit, and are thus arranged in a row in the X-axis direction. 
     The wheel cylinder ports  872  are opened in the top surface  803 . Thus, the space on the front surface  801  can be saved compared with a case in which the wheel cylinder ports  872  are opened in the front surface  801 , and the recessed parts  807  and  808  can easily be formed at the corners of the housing  8 . The wheel cylinder ports  872  are formed on the negative side in the Y-axis direction on the top surface  803 . Thus, the connection between the wheel cylinder ports  872  and the SOL/V IN accommodating holes  842  and the like is simplified by forming the wheel cylinder ports  872  in the electromagnetic valve region γ, while the interference between the wheel cylinder ports  872  and the cylinder accommodating ports  82  is avoided, thereby being capable of simplifying the oil passages. The four wheel cylinder ports  872  are arranged in a row in the X-axis direction on the negative side in the Y-axis direction on the top surface  803 . Thus, an increase in dimension in the Y-axis direction of the housing  8  can be suppressed by forming the wheel cylinder ports  872  in the single row in the Y-axis direction. 
     The master cylinder ports  871  are opened in the front surface  801 . Thus, the space on the top surface  803  can be saved compared with a case in which the master cylinder ports  871  are opened in the top surface  803 , and the wheel cylinder ports  872  and the like can easily be formed at the top surface  803 . The master cylinder ports  871 P and  871 S are on both sides of the reservoir chamber  830  in the X-axis direction (as viewed in the Y-axis direction). The reservoir chamber  830  is arranged between the ports  871 P and  871 S in the X-axis direction. The area of the front surface  801  can be decreased by using a space between the ports  871 P and  871 S to form the reservoir chamber  830  in this way, thereby decreasing the size of the housing  8 . The ports  871 P and  871 S are formed respectively between the reservoir chamber  830  and the cylinder accommodating holes  82 A and  82 E in the circumferential direction of the axial center O (as viewed in the Y-axis direction). Thus, an increase in dimension from the axial center O to the outer surface (top surface  803 ) of the housing  8  can be suppressed, thereby being capable of decreasing the size of the housing  8 . Moreover, the openings of the ports  871  on the front surface  801  can be formed on the center side in the X-axis direction, thereby being capable of forming the recessed parts  807  and  808  on the outer sides in the X-axis direction with respect to the ports  871 P and  871 S. The ports  871 P and  871 S open in a portion other than the motor housing  200  (flange part  203 ) on the front surface  801 . The ports  871 P and  871 S are on both sides with respect to the bolt hole  891  as viewed in the Y-axis direction. The openings of the ports  871 P and  871 S and the opening of the bolt hole  891  partially overlap with each other in the Z-axis direction (as viewed in the X-axis direction). Thus, an increase in dimension in the Z-axis direction of the front surface  801  can be suppressed. In other words, an area (on the positive side in the Z-axis direction with respect to the motor housing  200 ) of a portion in which the ports  871 P and  871 S are formed can be decreased on the front surface  801 , thereby being capable of decreasing the size of the housing  8 . 
     The suction port  873  is opened on the center side in the Y-axis direction in the top surface  803 . Thus, the suction port  873  can be formed between the electromagnetic valve region γ and the pump region β. Therefore, the suction port  873  (reservoir chamber  830 ) can easily be connected to both the valve body accommodating holes  84  and the cylinder accommodating holes  82  (suction ports  823  of the pump  3 ), thereby being capable of simplifying the oil passages. The suction port  873  is opened on the center side in the X-axis direction in the top surface  803 . Thus, when the one reservoir  120  is used in common for both the P and S systems, the suction port  873  (reservoir chamber  830 ) can easily be connected to the valve body accommodating holes  84 P and  84 S in both the systems, thereby being capable of simplifying the oil passages. 
     The wheel cylinder ports  872   c  and  872   d  are on both sides with respect to the suction port  873  (reservoir chamber  830 ), and the openings of the ports  872   c  and  872   d  and the suction port  873  (reservoir chamber  830 ) partially overlap with each other in the X-axis direction (as viewed in the Y-axis direction). Thus, an increase in dimension in the X-axis direction of the housing  8  can be suppressed, thereby being capable of decreasing the size. The openings of the ports  872   c  and  872   d  and the suction port  873  partially overlap with each other in the Y-axis direction (as viewed in the X-axis direction). Thus, an increase in dimension in the Y-axis direction of the top surface  803  can be suppressed. In other words, the area of a portion (on the positive side in the Y-axis direction with respect to the ports  872   c  and  872   d  or on the positive side in the Y-axis direction with respect to the electromagnetic valve region γ) in which the suction port  873  is formed can be decreased on the top surface  803 , thereby being capable of decreasing the size of the housing  8 . The cylinder accommodating holes  82 A and  82 E are on both the sides of the suction port  873  in the X-axis direction (as viewed in the Y-axis direction), and the openings of the holes  82 A and  82 E and the suction port  873  partially overlap with each other in the Y-axis direction (as viewed in the X-axis direction). Thus, the increase in dimension in the Y-axis direction of the top surface  803  can be suppressed. In other words, the area of a portion (on the negative side in the Y-axis direction with respect to the ports  82 A and  82 E or on the negative side in the Y-axis direction with respect to the pump region β) in which the suction port  873  is formed can be decreased on the top surface  803 , thereby being capable of decreasing the size of the housing  8 . 
     The reservoir chamber  830  is formed in the region between the cylinder accommodating holes  82 A and  82 E which are adjacent to each other, in the circumferential direction of the axial center O. Thus, the increase in dimension from the axial center O to the outer surface (top surface  803 ) of the housing  8  extending along the circumferential direction of the axial center O can be suppressed, thereby being capable of decreasing the size of the housing  8 . Moreover, the oil passages connecting the reservoir chamber  830  and the suction ports  823  of the pump  3  to each other can be shortened. The cylinder accommodating holes  82 A and  82 E and the reservoir chamber  830  partially overlap with each other in the Y-axis direction (as viewed in the X-axis direction). Thus, the increase in dimension in the Y-axis direction of the housing  8  can be suppressed, thereby being capable of decreasing the size. The reservoir chamber  830  is arranged in the region surrounded by the master cylinder ports  871 P and  871 S and the wheel cylinder ports  872   c  and  872   d . The size of the housing  8  can be decreased by using the space between the respective ports to form the reservoir chamber  830  in this way. 
     The back pressure port  874  is opened in the right side surface  805 . Thus, a space on the front surface  801  or the top surface  803  can be saved compared with a case in which the back pressure port  874  is opened in the front surface  801  or the top surface  803 . Therefore, an increase in the area of the front surface  801  or the top surface  803  can be suppressed, thereby suppressing the increase in size of the housing  8 . The back pressure port  874  is formed on the negative side in the Z-axis direction of the right side surface  805 . Thus, the back pressure port  874 , and the SS/V IN  27  and SS/V OUT  28  are easily connected to each other by forming the back pressure port  874  close to the SS/V IN accommodating hole  847  and the SS/V OUT accommodating hole  848  in the Z-axis direction, thereby simplifying the oil passages. The back pressure port  874  may be opened in the left side surface  806 . In this embodiment, the back pressure port  874  is opened in the right side surface  805 . The connector part  903  is not adjacent to the right side surface  805 . Thus, compared with a case in which the back pressure port  874  is adjacent to the left side surface  806 , the interference between the connector (harness) connected to the connector part  903  and the pipe  10 X connected to the back pressure port  874  can be suppressed. In other words, when the back pressure port  874  is connected to the pipe  10 X, the connection can easily be carried out. Thus, the mounting workability of the braking system  1  in the vehicle can be increased. 
     (Suppression of Vibration and Improvement in Support Rigidity) 
     The housing  8  (second unit  1 B) is fixed to the vehicle body side via the mount  102 . Thus, supportability of the structure configured to support the housing  8  can be improved. Moreover, a rotation force of the motor  20  acts as a reaction force on the motor housing  200  and the housing  8  via bearings of the motor rotation shaft and the pump rotation shaft  300 . Vibration occurs mainly in the circumferential direction of the axial center O in the second unit  1 B by the reaction force during operation of the motor  20  (pump  3 ). The housing  8  (second unit  1 B) is supported by the vehicle body side (mount  102 ) via the insulators  103  and  104 . The insulators  103  and  104  are configured to absorb the vibration generated by the operation of the second unit  1 B. As a result, transmission of the vibration from the second unit  1 B to the vehicle body side via the mount  102  is suppressed. Thus, silence of the braking system  1  can be achieved. 
     The second unit  1 B can stably be held by supporting the bottom surface  804  and the front surface  801  of the housing  8  at the four locations as follows. The bolt holes  895  are opened in the bottom surface  804 . Thus, the second unit  1 B can stably be supported with respect to the vehicle body side (mount  102 ) by the bolts B 3  fixed to the bolt holes  895  receiving the weight (load in the vertical direction) of the second unit  1 B in axial directions of the bolts B 3 . The bolt holes  894  are opened in the front surface  801 . The center of gravity of the second unit  1 B is displaced to the front surface  801  side with respect to the center of gravity of the housing  8  due to the mounting of the motor  20 . The second unit  1 B is caused to fall toward the front surface  801  side due to the weight of the motor  20 . The second unit  1 B can stably be supported with respect to the vehicle body side (mount  102 ) by the bolts B 4  fixed into the bolt holes  894  receiving the load in the falling direction of the second unit  1 B in axial directions of the bolts B 4 . The bolt holes  894  are formed on the negative side in the Z-axis direction on the front surface  801 . Thus, the size of an arm part of the mount  102  can be decreased, thereby being capable of improving mountability of the braking system  1 . 
     The two bolt holes  895  are opened in the bottom surface  804 . Thus, the second unit  1 B can more stably be supported by supporting the housing  8  at the two points. Moreover, a load acting on each of the bolt holes  895  can be decreased by distributing the load of the second unit  1 B to the two bolt holes  895  (bolts B 3 ) for support. Dimensions of each of the bolt holes  895  can be decreased, thereby being capable of decreasing the size of the housing  8 . The center of gravity of the second unit  1 B is located on the center side in the X-axis direction (on the side closer to the axial center O). The two bolt holes  895  are formed on the both sides in the X-axis direction with respect to the axial center O on the bottom surface  804 . Thus, the second unit  1 B can more stably be supported by fixing the housing  8  on the both sides with respect to the center of gravity. Moreover, the vibration of the second unit  1 B in the circumferential direction of the axial center O can effectively be suppressed by fixing the housing  8  at the plurality of positions separated in the circumferential direction of the axial center O. The two bolt holes  895  are formed at the ends on the both sides in the X-axis direction on the bottom surface  804 . Thus, the second unit  1 B can more stably be supported by increasing the distance between the support points. Moreover, the load acting on the bolt hole  895  can be decreased by increasing the distance in the X-axis direction from the center of gravity of the second unit  1 B to the bolt hole  895 . Similarly, the two bolt holes  894  are opened in the front surface  801 . The two bolt holes  894  are formed on the both sides in the X-axis direction with respect to the axial center O. The bolt holes  894  are formed at the ends on the both sides in the X-axis direction on the front surface  801 . Thus, the bolt holes  894  respectively provide the same actions and effects as described above. The axial center of each of the bolt holes  894  is in the X-axis direction, and is arranged so as to be separated more from the axial center O than the axial center of each of the bolt holes for the motor mounting, on the front surface  801 . Thus, the second unit  1 B can more stably be supported by increasing the distance between the support points. 
     The external devices (the master cylinder  5 , the wheel cylinders W/C, and the stroke simulator  6 ) are connected to the housing  8  by the pipes  10 M,  10 W, and  10 X. The housing  8  can efficiently be supported through the pipes  10 M,  10 W, and  10 X. The external device may be separately outside the second unit  1 B, and may be, for example, a hydraulic pressure unit including a second pump (third hydraulic pressure source) other than the third pump, a second motor configured to drive the second pump, an ECU configured to control the number of revolutions of the second motor, and the like. In this case, the second pump is connected to the second unit  1 B by a pipe, and can supply a hydraulic pressure to the second unit  1 B. A port of the second unit  1 B to which the pipe is connected is opened, for example, on the right side surface  805  like the back pressure port  874 , and is connected to the supply oil passages inside the housing  8 . The brake fluid discharged from the second pump is supplied to the supply oil passages  11  via the pipe. 
     Each of the pipes  10 M,  10 W, and  10 X is a metal pipe, and has rigidity equivalent to that of the mount  102 . A support structure constructed of the pipes  10 M,  10 W, and  10 X can have the rigidity equivalent to that of the mount  102 . The respective pipes  10 M,  10 W, and  10 X can increase support rigidity for the housing  8 . For example, when the sensors (for example, an angular velocity sensor) configured to detect the motion state of the vehicle are mounted to the control board  900 , misdetection of the vibration as the motion (yaw rate and the like) of the vehicle body can be suppressed by suppressing the vibration of the second unit  1 B. Moreover, the sizes of the insulators  103  and  104  can be decreased, thereby improving the mountability of the braking system  1 . The respective pipes  10 M,  10 W, and  10 X bend a plurality of times. The rigidity of the metal pipe increases after the bending. The support rigidity for the housing  8  by the respective pipes  10 M,  10 W, and  10 X can be increased by bending the respective pipes  10 M,  10 W, and  10 X a plurality of times. For example, the back pressure pipe  10 X bends a plurality of times between the first unit  1 A and the back pressure port  874 . Thus, the support rigidity for the housing  8  by the back pressure pipe  10 X can be increased. 
     The two master cylinder ports  871 , the four wheel cylinder ports  872 , and the one back pressure port  874  are formed on the housing  8 , and the pipes  10 MP,  10 MS,  10 W (FL),  10 W (RR),  10 W (FR),  10 W (FR), and  10 X are respectively connected to those ports. The supportability for the housing  8  can be increased by supporting the housing  8  at a total of seven portions by the pipes in this way. The master cylinder pipes  10 M and the wheel cylinder pipes  10 W are connected on the positive side in the Z-axis direction to the housing  8 , and the back pressure pipe  10 X is connected on the negative side in the Z-axis direction to the housing  8 , with respect to the axial center O. Thus, the supportability for the housing  8  by the respective pipes  10 M,  10 W, and  10 X can be increased by connecting the pipes  110 M,  10 W, and  10 X to the housing  8  on the both sides in the Z-axis direction with respect to the axial center O. 
     The master cylinder ports  871  are opened in the front surface  801 . Thus, the second unit  1 B can stably be supported with respect to the vehicle body side by the pipes  10 M fixed into the master cylinder ports  871  receiving the load in the falling direction of the second unit  1 B in axial directions of the pipes  10 M, like the bolts B 4  on the front surface  801 . The master cylinder ports  871  are formed on the positive side in the Z-axis direction with respect to the axial center O. Thus, the load in the falling direction can efficiently be received by the master cylinder pipes  10 M, and the second unit  1 B can thus stably be supported. Moreover, the housing  8  can be fixed at the positions on the both sides of the center of gravity of the second unit  1 B by the bolts B 4  (on the negative side in the Z-axis direction with respect to the axial center O) and the master cylinder pipes  10 M on the front surface  801 . Therefore, the second unit  1 B can more stably be supported. Moreover, the vibration of the second unit  1 B in the circumferential direction of the axial center O may be transmitted to the first unit  1 A via the metal pipes (master cylinder pipes  10 M and the back pressure pipe  10 X), and may further be transmitted to the dash panel on the vehicle body side via the flange part  78 . Noise may occur in the vehicle cabin as a result of the transmission of the vibration to the dash panel. The two master cylinder ports  871 P and  871 S are arranged in a row in the X-axis direction. Thus, the vibration of the second unit  1 B can effectively be suppressed by fixing the housing  8  through the pipes  10 M at the plurality of positions separated in the circumferential direction of the axial center O. As a result, the vibration transmitted to the vehicle body side via the first unit  1 A (flange part  78 ) can be decreased, thereby being capable of achieving the silence in the vehicle cabin. 
     The wheel cylinder ports  872  are opened in the top surface  803 . Thus, the pipes  10 W fixed to the wheel cylinder ports  872  pull the housing  8  in their axial direction (to the positive side in the Z-axis direction), and receive the load of the second unit  1 B, thereby enabling stable support for the second unit  1 B with respect to the vehicle body side. The wheel cylinder ports  872  are formed on the positive side in the Z-axis direction with respect to the axial center O. Thus, the housing  8  is fixed at the positions on the both sides of the center of gravity of the second unit  1 B by the bolts B 3  on the bottom surface  804  and the wheel cylinder pipes  10 W. Thus, the second unit  1 B can more stably be supported. Moreover, the four wheel cylinder ports  872  are arranged in a row in the X-axis direction. Thus, the vibration of the second unit  1 B in the circumferential direction of the axial center O can effectively be suppressed by fixing the housing  8  at the plurality of positions separated in the circumferential direction of the axial center O. Particularly, the wheel cylinder ports  872  are opened in the top surface  803 , which is a surface along the circumferential direction of the axial center O. The vibration of the second unit  1 B in the circumferential direction of the axial center O can more effectively be suppressed by the tensile forces of the wheel cylinder pipes  10 W acting on the housing  8  in the direction away from the axial center O. 
     The back pressure port  874  is opened in the right side surface  805 . Thus, the pipe  10 X fixed into the back pressure port  874  pulls the housing  8  in its axial direction (to the positive side of the X axis) to receive the load of the second unit  1 B, to thereby enable stable support for the second unit  1 B with respect to the vehicle body side. The back pressure port  874  is formed on the negative side in the Z-axis direction with respect to the axial center O. Thus, the housing  8  is fixed at the positons on the both sides of the center of gravity of the second unit  1 B by the master cylinder pipes  10 M and the wheel cylinder pipes  10 W on the positive side in the Z-axis direction with respect to the axial center O and the back pressure pipe  10 X in the negative side in the Z-axis direction. Thus, the second unit  1 B can more stably be supported. Moreover, distances between the master cylinder pipes  10 M and the wheel cylinder pipes  10 W, and the back pressure pipe  10 X are long in the circumferential direction of the axial center O. Thus, the vibration of the second unit  1 B in the circumferential direction of the axial center O can effectively be suppressed by increasing the distances between the fixing positions of the housing  8  in the circumferential direction of the axial center O. Particularly, the back pressure port  874  is opened in the right side surface  805 , which is a surface along the circumferential direction of the axial center O. The vibration of the second unit  1 B in the circumferential direction of the axial center O can more effectively be suppressed by the tensile force of the back pressure pipe  10 X acting on the housing  8  in the direction away from the axial center O. The vibration of the second unit  1 B in the circumferential direction of the axial center O can more effectively be suppressed by arranging the action points of the tensile forces by the wheel cylinder pipes  10 W and the action point of the tensile force by the back pressure pipe  10 X on both sides in the Z-axis direction with respect to the axial center O. 
     Second Embodiment 
     First, a description is given of a configuration. The housing  8  of the second embodiment includes two liquid reservoir chambers  832 .  FIG. 23  and  FIG. 24  are views for illustrating passages, recessed parts, and holes in this embodiment with transparently in the housing  8 .  FIG. 23  is a front transparent view similar to  FIG. 4 .  FIG. 24  is a transparent view for illustrating the housing  8  as viewed from the positive side of the X axis, the positive side of the Y axis, and the negative side in the Z-axis direction. The two liquid reservoir chambers  832  are provided on the both sides in the X-axis direction with respect to the axial center O so as to sandwich the cylinder accommodating hole  82 C, and are opened in the bottom surface  804 . Each of the liquid reservoir chambers  832  is connected to the cam accommodating hole  81  via the oil passage hole  881 . Each of the liquid reservoir chambers  832  is smaller in volume of the small-diameter part  832   s  and the medium-diameter part  832   m , and smaller in dimension in the Z-axis direction than that of the first embodiment. The eighth hole  88 - 48  of the fourth hole group  88 - 4  is provided on the opposite side of that of the first embodiment in the X-axis direction with respect to the axial center O. As illustrated as broken lines of  FIG. 23 , lid members  832   a  close the openings of the liquid reservoir chambers  832 , and protrude from the bottom surface  804 . A sum of the volume of the liquid reservoir chamber  832  and the volume of the lid member  832   a  is a substantial capacity of the liquid reservoir chamber  832 . The lid member  832   a  is provided so that its position in the Z-axis direction is adjustable with respect to the housing  8  (bottom surface  804 ) by means of, for example, a thread or the like, to thereby enabling a change in substantial capacity of the liquid reservoir chamber  832 . Other configurations are the same as that of the first embodiment. 
     A description is now given of actions and effects. Compared with the first embodiment, the volume of each of the liquid reservoir chambers  832  is smaller inside the housing  8 , but a large capacity can be secured as a whole by providing the two liquid reservoir chambers  832 . Moreover, the capacity of the liquid reservoir chamber  832  can be adjusted by adjusting the position in the Z-axis direction of the lid member  832   a  in accordance with a required amount of the liquid for the liquid reservoir chamber  832 . The number of the liquid reservoir chambers  832  is not limited to two. The other actions and effects are the same as those of the first embodiment. 
     Other Embodiments 
     The embodiments of the present invention have been described above based on the drawings. However, the specific configuration of the present invention is not limited to the configuration described in each of the embodiments. A change in design or the like without departing from the scope of the gist of the invention is encompassed in the present invention. Further, within a range in which the above-mentioned problems can be at least partially solved or within a range in which the above-mentioned effects are at least partially obtained, a suitable combination or omission of the components recited in the claims and described in the specification is possible. 
     The present application claims priority to the Japanese Patent Application No. 2015-163109 filed on Aug. 20, 2015. The entire disclosure including the specification, the claims, the drawings, and the abstract of Japanese Patent Application No. 2015-163109 filed on Aug. 20, 2015 is incorporated herein in its entirety by reference. 
     REFERENCE SIGNS LIST 
       1  braking system,  1 A first unit (master cylinder unit),  1 B second unit (hydraulic pressure control unit),  10 X back pressure pipe,  11  supply oil passage (brake oil passage, brake fluid passage),  120  reservoir,  16  back pressure oil passage (brake oil passage, brake fluid passage),  17  first simulator oil passage (brake oil passage, brake fluid passage),  20  motor,  27  SS/V IN (electromagnetic valve, switch part),  270  check valve (switch part),  28  SS/V OUT (electromagnetic valve, switch part),  3  pump (rotational pump),  301  cam (eccentric cam),  36  piston (plunger),  5  master cylinder,  6  stroke simulator,  601  positive pressure chamber (one chamber, first chamber),  602  back pressure chamber (another chamber, second chamber),  61  piston,  71  cylinder,  8  housing,  801  front surface (mounting surface),  90   f  sudden brake operation state determination part, W/C wheel cylinder, β pump region (pump part), γ electromagnetic valve region (electromagnetic valve part)