Patent Publication Number: US-2005115236-A1

Title: Master cylinder with a braking stroke simulator

Description:
This application claims priorities under 35 U.S.C. Sec.119 to Nos. 2003-386661 filed in Japan on Nov. 17, 2003, 2003-386662 filed in Japan on Nov. 17, 2003, and 2004-131804 filed in Japan on Apr. 27, 2004, the entire contents of which are herein incorporated by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a master cylinder for use in a hydraulic brake apparatus of a vehicle, and more particularly to a master cylinder with a braking stroke simulator operated in response to operation of a manually operated braking member.  
      2. Description of the Related Arts  
      Heretofore, there is known various hydraulic brake apparatuses each having a master cylinder with a braking stroke simulator. Among them, such an apparatus as discussed below has been disclosed in Japanese Patent Laid-open publication No.11-59349. According to the apparatus, when a pressure control device including a pressure source is normal, the hydraulic pressure generated by the pressure source is controlled by the pressure control device in response to operation of a manually operated braking member to be supplied into wheel brake cylinders, with the communication between the master cylinder and the wheel brake cylinder being blocked. When the pressure control device has come to be abnormal, the master cylinder is communicated with the wheel brake cylinder, to discharge the hydraulic pressure into the wheel brake cylinder in response to operational force of the manually operated braking member.  
      In general, the stroke simulator is adapted to provide the manually operated braking member with a stroke in response to the braking operation force, when the pressure control device is normal, i.e., when the communication between the master cylinder and the wheel brake cylinder has been blocked. And, according to the hydraulic brake apparatus as disclosed in the Japanese Patent Laid-open publication, the stroke simulator is disposed between the manually operated braking member and a master piston. In view of the fact that it is required to provide a large stroke of a brake pedal in response to a stroke of the stroke simulator, when the pressure control device is abnormal, i.e., when the hydraulic pressure is supplied from the master cylinder to the wheel brake cylinder, there is provided cut-off means for blocking the communication between a simulator chamber and an atmospheric pressure chamber in response to movement of the master piston. As for the cut-off means, there are provided a sleeve in contact with a part of inner surface of a cylinder body, and a seal member fixed to the master piston, whereby the stroke of the stroke simulator may be restricted, when the hydraulic pressure is supplied from the master cylinder to the wheel brake cylinder.  
      According to the hydraulic brake apparatus as disclosed in the Japanese Patent Laid-open publication, however, if the pressure control device became abnormal, the seal member would close a port formed on the sleeve, to block the communication between the simulator chamber and the atmospheric pressure chamber. Therefore, the sleeve is required to serve as the cut-off means, so that the apparatus costs much. Otherwise, if the sleeve was omitted from the apparatus as disclosed in the Japanese Patent Laid-open publication, and instead the port was formed directly on the cylinder body, communication passages would be complicated, so that the apparatus would cost much, as well.  
      Also, if the pressure control device becomes abnormal for example, it is desirable to block the communication between the communication between the simulator chamber and the atmospheric pressure chamber, when the master piston is advanced slightly over a so-called port idle for blocking the communication between the communication between the master pressure chamber and the atmospheric pressure chamber, in order to reduce a stroke of the stroke simulator as small as possible. Therefore, a high dimensional accuracy is required for positioning ports formed on the master piston, grooves for holding the seal members, and ports formed on the sleeve, so that the apparatus would cost much. According to the hydraulic brake apparatus as disclosed in the Japanese Patent Laid-open publication, the port idle will cause a so-called dead stroke, which will result in increasing the stroke of the manually braking member when the pressure control device becomes abnormal. In order to reduce the size of the port idle, therefore, a high dimensional accuracy is required for setting dimensions of the cylinder housing, a cup-like spring holder, and axial members or the like, so that the apparatus would cost much, as well.  
     SUMMARY OF THE INVENTION  
      Accordingly, it is an object of the present invention to provide a master cylinder having a braking stroke simulator used for a component of a hydraulic brake apparatus for a vehicle, which is capable of restricting a stroke of a manually operated braking member when the hydraulic pressure is supplied from the master cylinder to wheel brake cylinders.  
      And, it is another object of the present invention to provide an inexpensive apparatus provided with a master cylinder having a braking stroke simulator, which is capable of blocking the communication between a simulator chamber and an atmospheric pressure chamber appropriately, when a master piston is advanced.  
      In order to accomplish the above and other objects, the master cylinder is provided with a piston member which is slidably accommodated in a cylinder bore of a cylinder housing for defining a master pressure chamber in front of the piston member, and a stroke simulator which has a simulator piston for defining a simulator chamber in front of the simulator piston and moving back and forth in response to operation of a manually operated braking member, and an elastic member for applying a stroke of the simulator piston in response to braking operation force of the manually operated braking member. The stroke simulator is adapted to transmit the braking operation force of the manually operated braking member to the piston member, through the simulator piston and the elastic member. Furthermore, a communication control device is provided for communicating the simulator chamber with the atmospheric pressure chamber when the piston member is placed in an initial position thereof, and blocking the communication between the simulator chamber and the atmospheric pressure chamber in response to movement of the piston member advanced from the initial position thereof. The communication control device includes a seal member mounted on one of the piston member and the inner surface of the cylinder bore. The communication control device is adapted to communicate the simulator chamber with the atmospheric pressure chamber, in response to a first relative relationship of the seal member with the other one of the piston member and the inner surface of the cylinder bore, when the piston member is placed in the initial position thereof, and adapted to block the communication between the simulator chamber and the atmospheric pressure chamber, in response to a second relative relationship of the seal member with the other one of the piston member and the inner surface of the cylinder bore, when the piston member is advanced from the initial position thereof by a predetermined distance or more.  
      In the master cylinder with the braking stroke simulator as described above, the communication control device may include an annular groove formed on the inner surface of the cylinder bore with a certain width along a longitudinal axis of the cylinder bore, and the seal member mounted around the piston member. The communication control device is adapted to communicate the simulator chamber with the atmospheric pressure chamber through a clearance between the seal member and the annular groove, when the piston member is placed in the initial position thereof, and adapted to block the communication between the simulator chamber and the atmospheric pressure chamber, with the seal member being placed to contact the inner surface of the cylinder bore, when the piston member is advanced from the initial position thereof by the predetermined distance or more.  
      The communication control device may include the seal member mounted on the inner surface of the cylinder bore, and a small diameter portion and a large diameter portion formed around the piston member. The communication control device is adapted to communicate the simulator chamber with the atmospheric pressure chamber through a clearance between the seal member and the small diameter portion, when the piston member is placed in the initial position thereof, and adapted to block the communication between the simulator chamber and the atmospheric pressure chamber, with the seal member being placed to contact the large diameter portion, when the piston member is advanced from the initial position thereof by the predetermined distance or more.  
      Or, the communication control device may include the seal member mounted on the inner surface of the cylinder bore, and a communication passage formed on the piston member. The communication control device is adapted to communicate the simulator chamber with the atmospheric pressure chamber through the communication passage, when the piston member is placed in the initial position thereof, and adapted to block the communication between the simulator chamber and the atmospheric pressure chamber, with the seal member being placed to close the communication passage, when the piston member is advanced from the initial position thereof by the predetermined distance or more.  
      Preferably, the communication passage is at least a communication hole formed in the piston member in a radial direction thereof. The communication passage may be at least a communication groove formed on the piston member in a longitudinal direction thereof. Or, the communication passage may be at least a cut-out portion formed around a part of the outer peripheral surface of the piston member.  
      In order to accomplish another object as described above, particularly, the master cylinder may include a master piston which is slidably accommodated in a cylinder bore of a cylinder housing for defining a master pressure chamber in front of the master piston, and a stroke simulator which has a simulator piston for defining a simulator chamber in front of the simulator piston and moving back and forth in response to operation of the manually operated braking member, to communicate the master pressure chamber with the atmospheric pressure chamber when the master piston is placed in an initial position thereof, and block the communication between the master pressure chamber and the atmospheric pressure chamber when the master piston is advanced from the initial position thereof by a first stroke or more. The stroke simulator has an elastic member for applying a stroke of the simulator piston in response to braking operation force of the manually operated braking member. And, the stroke simulator is adapted to transmit the braking operation force of the manually operated braking member to the master piston, through the simulator piston and the elastic member. Furthermore, a communication control device is provided for communicating the simulator chamber with the atmospheric pressure chamber when the master piston is placed in the initial position thereof, and blocking the communication between the simulator chamber and the atmospheric pressure chamber in response to movement of the master piston. And, the communication control device includes an auxiliary piston which is disposed between the master piston and the elastic member, and adapted to block the communication between the simulator chamber and the atmospheric pressure chamber when the auxiliary piston is advanced from the initial position thereof by a second stroke or more. The second stroke is set to be greater than the first stroke by a predetermined distance.  
      Preferably, the master cylinder further includes a cut-off stroke setting device which is disposed between the master piston and the auxiliary piston for adjusting a distance between the master piston and the auxiliary piston to set the predetermined distance, and which may include a rod disposed between the master piston and the auxiliary piston for adjusting the distance between them.  
      The master cylinder may further include a port idle setting device for adjusting an initial position of at least one of the master piston and the auxiliary piston to set the first stroke. The port idle setting device may include a stopper secured to the housing for adjusting the initial position of at least one of the master piston and the auxiliary piston.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above stated object and following description will become readily apparent with reference to the accompanying drawings, wherein like reference numerals denote like elements, and in which:  
       FIG. 1  is a sectional view of a master cylinder with a braking stroke simulator according to an embodiment of the present invention;  
       FIG. 2  is a schematic block diagram of a hydraulic brake apparatus having a master cylinder with a braking stroke simulator according to an embodiment of the present invention;  
       FIG. 3  is a sectional view of a master cylinder with a braking stroke simulator according to another embodiment of the present invention;  
       FIG. 4  is a sectional view of a master cylinder with a braking stroke simulator according to a further embodiment of the present invention;  
       FIG. 5  is a sectional view of a master cylinder with a braking stroke simulator according to a yet further embodiment of the present invention;  
       FIG. 6  is a sectional view of a master piston, with a tip end portion of its large diameter portion sectioned in a direction perpendicular to its longitudinal axis according to the embodiment as shown in  FIG. 5 ;  
       FIG. 7  is a sectional view of another master piston, with a tip end portion of its large diameter portion sectioned in a direction perpendicular to its longitudinal axis according to the embodiment as shown in  FIG. 5 ;  
       FIG. 8  is a sectional view of a master cylinder with a braking stroke simulator according to a yet further embodiment of the present invention;  
       FIG. 9  is a sectional view of a part of a master cylinder with a braking stroke simulator having a rod served as an embodiment of a cut-off stroke setting device for use in a communication control device according to the present invention;  
       FIG. 10  is a sectional view of a part of a master cylinder with a braking stroke simulator having an adjusting rod served as an embodiment of a cut-off stroke setting device according to the present invention;  
       FIG. 11  is a sectional view of a part of a master cylinder with a braking stroke simulator having an adjusting rod served as another embodiment of a cut-off stroke setting device according to the present invention;  
       FIG. 12  is a sectional view of a part of a master cylinder with a braking stroke simulator having an adjusting rod served as a further embodiment of a cut-off stroke setting device according to the present invention;  
       FIG. 13  is a sectional view of a part of a master cylinder with a braking stroke simulator having a stopper rod served as an embodiment of a port idle setting device according to the present invention;  
       FIG. 14  is a sectional view of a part of a master cylinder with a braking stroke simulator having a stopper rod served as another embodiment of a port idle setting device according to the present invention;  
       FIG. 15  is a sectional view of a portion to be screwed with the stopper as shown in  FIG. 8  and a portion to be screwed with the adjusting rod as shown in  FIG. 10 ; and  
       FIG. 16  is a schematic block diagram of a hydraulic brake apparatus having a master cylinder with a braking stroke simulator according to the embodiment as shown in  FIG. 8 .  
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Referring to  FIG. 1 , there is illustrated a master cylinder MC with a stroke simulator SM formed in a body according to an embodiment of the present invention, which includes a master piston MP served as a piston member of the present invention and slidably accommodated in a cylinder housing HS, with a simulator piston SP slidably accommodated in the master piston MP. The housing HS is closed in its front end (leftward in  FIG. 1 ) to be formed in a cylinder with a bottom, with a cylinder bore having a stepped bore of a recess B 1 , a small diameter bore B 2  and a large diameter bore B 3 . At the rear end of the housing HS, there is formed an open end portion B 4  with threaded grooves formed therein. On the inner surface of the small diameter bore B 2 , an annular groove G 1  is formed for holding a seal member S 5  having a cup-like cross section, whereas on the inner surface of the large diameter bore B 3 , there is formed an annular groove G 2  having a certain width along the longitudinal axis of the bore B 3 . On the side wall of the housing HS, there are formed a port P 1  opening into the recess B 1 , and a port P 2  opening into the large diameter bore B 3  near the small diameter bore B 2 . The housing HS may be made of a single metallic member, because those recess B 1 , small diameter bore B 2 , large diameter bore B 3 , open end portion B 4 , and annular grooves G 1  and G 2  can be formed by boring the housing HS along the longitudinal axis thereof.  
      As for the master piston MP, there are formed at its front end a recess M 1  opening forward, and formed at its rear end a recess opening rearward, in the latter of which a cylinder bore is formed to provide a stepped bore of a small diameter bore M 2  and a large diameter bore M 3 . On the inner surface of the large diameter bore M 3  near the open end thereof, an annular groove MG is formed for holding a C-ring CR as described later. On the side wall of the master piston MP, there are formed a port P 3  opening into the recess M 1 , and a port P 4  opening into the small diameter bore M 2 . A land portion L 1  is formed around the outer peripheral surface of a middle portion of the master piston MP, and a land portion L 2  is formed around the outer peripheral surface of its rear portion, with annular grooves formed on their outer surfaces, to hold therein annular seal members S 2  and S 3  having cup-like cross sections, respectively.  
      The simulator piston SP has a large diameter piston portion SP 1  to be slidably accommodated in the large diameter bore M 3 , and a small diameter axial portion SP 2  extending rearward from the former. On the outer peripheral surface of the piston portion SP 1 , there is formed an annular groove for holding therein an annular seal member S 4  having a cup-like cross section. The axial portion SP 2  is connected to a brake pedal BP served as the manually operated braking member. The seal members S 1  and S 2  act as a check valve, respectively, to block the flow of brake fluid from the opened side of cup-like cross section to the closed side thereof, and allow the flow of brake fluid from the closed side to the opened side, so that the seal member S 2  allows the flow of brake fluid from the front side (left side in  FIG. 1 ) to the rear side, and blocks its reverse flow.  
      Next will be explained the parts as described above, according to an example of a sequence of steps for assembling them. At the outset, a compression spring E 2  served as an elastic member for the simulator is received into the small diameter bore M 2  and large diameter bore M 3  of the master piston MP. Then, the simulator piston SP with the seal member S 4  mounted thereon is fluid-tightly and slidably received into the large diameter bore M 3  to define a simulator chamber C 4  in front of the piston portion SP 1 . With the piston portion SP 1  accommodated in the large diameter bore M 3 , the C-ring CR is fitted into the annular groove MG of the master piston MP, to prevent the simulator piston SP from being moved rearward against biasing force of the compression spring E 2 . Then, the seal members S 2  and S 3  are mounted on the land portions L 1  and L 2  of the master piston MP, respectively.  
      Next, the seal member S 1  is fitted into the annular groove G 1  of the housing HS, and a compression spring E 1  served as a return spring is received in the recess B 1  of the housing HS and the recess M 1  of the master piston MP, and then the master piston MP is fitted into the small diameter bore B 2  and large diameter bore B 3 . Consequently, the master piston MP is fluid-tightly and slidably accommodated in the small diameter bore B 2  and large diameter bore B 3 , through the seal members S 1  and S 3 , respectively. Thus, with the master piston MP accommodated in the small diameter bore B 2  and large diameter bore B 3  of the housing HS, screwed into the open end portion B 4  of the housing HS is a nut-like stopper NH with threaded grooves formed on its outer peripheral surface, which prevents the master piston MP from being moved rearward against the biasing force of the compression spring E 1 .  
      With those parts assembled as described above, the master pressure chamber C 1  is defined in front of the master piston MP in the master cylinder MC, to be communicated with the wheel brake cylinder WC through the port P 1  (via an electromagnetic switching valve NO as described hereinafter). An atmospheric pressure chamber C 2  is formed between the seal members S 1  and S 2  held on the inner peripheral surface of the housing HS, and an annular chamber C 3  is formed between the seal members S 2  and S 3 , so that the atmospheric pressure chamber C 2  is so constituted to be always communicated with an atmospheric pressure reservoir RS (hereinafter, simply referred to as a reservoir RS) through the port P 2 . When the master piston MP is placed in its initial position as shown in  FIG. 1 , therefore, the master pressure chamber C 1  is communicated with the atmospheric pressure chamber C 2  through the port P 3 , and finally communicated with the reservoir RS under the atmospheric pressure, through the port P 2 . On the contrary, when the master piston MP is advanced from its initial position by a first stroke and more, the opening area of the port P 3  is closed by the seal member S 1 , thereby to block the communication between the master pressure chamber C 1  and the atmospheric pressure chamber C 2  (and the reservoir RS). At the same time, when the master piston MP is placed in its initial position as shown in  FIG. 1 , the atmospheric pressure chamber C 2  is communicated with the annular chamber C 3  through the clearance CL between the seal member S 2  and the annular groove G 2 , and therefore the simulator chamber C 4  is communicated with the annular chamber C 3  and the atmospheric pressure chamber C 2  through the port P 4 , whereby the simulator chamber C 4  is communicated with the reservoir RS through the port P 2 . And, when the master piston MP is advanced from the initial position thereof by a second stroke, which is greater than the first stroke, or more, the communication between the annular chamber C 3  (then, the simulator chamber C 4 ) and the atmospheric pressure chamber C 2  will be blocked by the seal member S 2  and the inner surface of the large diameter bore B 3 . Thus, a communication control device according to the present invention is constituted.  
      The master cylinder with the braking stroke simulator as described above is provided to constitute a hydraulic brake apparatus for a vehicle as shown in  FIG. 2 , wherein the master pressure chamber C 1  of the master cylinder MC is connected to a wheel brake cylinder WC operatively mounted on each wheel of the vehicle through a normally open electromagnetic switching valve NO. And, a pressure source PG for generating a certain hydraulic pressure irrespective of the braking operation of the vehicle driver is connected to a hydraulic passage between the switching valve NO and the wheel brake cylinder WC.  
      According to the present embodiment, the pressure source PG includes an electric motor M controlled by an electronic control unit ECU, and a hydraulic pressure pump HP, which is driven by the electric motor M, and whose inlet is connected to the reservoir RS, and whose outlet is connected to an accumulator AC. According to the present embodiment, a pressure sensor Sps is connected to the outlet, and the detected pressure is monitored by the electronic control unit ECU. On the basis of the monitored result, the motor M is controlled by the electronic control unit ECU to keep the hydraulic pressure in the accumulator AC between predetermined upper and lower limits. The accumulator AC is connected to a hydraulic passage between the switching valve NO and the wheel brake cylinder WC, through a first linear solenoid valve SV 1  of a normally closed type, to regulate the hydraulic pressure discharged from the pressure source PG and supply it to the wheel brake cylinder WC. Also, the reservoir RS is connected to the hydraulic passage between the switching valve NO and wheel brake cylinder WC, through a second linear solenoid valve SV 2  of a normally closed type, to reduce the pressure in the wheel brake cylinder WC and regulate it. Accordingly, a pressure control device PC is formed by the pressure source PG, first and second linear solenoid valves SV 1  and SV 2 , electronic control unit ECU, and sensors as described hereinafter.  
      According to the present embodiment, a pressure sensor Smc is disposed in a hydraulic passage between the master cylinder MC and the switching valve NO, and a pressure sensor Swc is disposed in a hydraulic passage between the switching valve NO and the wheel brake cylinder WC. On the brake pedal BP, a stroke sensor BS is operatively connected to detect its stroke. The signals detected by the sensors as described above are fed to the electronic control unit ECU. Thus, the hydraulic braking pressure discharged from the master cylinder MC, the hydraulic braking pressure in the wheel brake cylinder WC and the stroke of the brake pedal BP are monitored by those sensors. Furthermore, in order to achieve those controls including an anti-skid control or the like, sensors SN such as wheel speed sensors, acceleration sensor or the like have been provided, so that the signals detected by them are fed to the electronic control unit ECU.  
      Hereinafter, explained is operation of the hydraulic brake apparatus having the master cylinder MC with the braking stroke simulator SM as constituted above. At the outset, when the pressure control device PC is normal, the switching valve NO is energized to be placed in its closed position, so that the communication between the master cylinder MC and the wheel brake cylinder WC is blocked, and the hydraulic pressure discharged from the master cylinder MC is supplied to the wheel brake cylinder WC in response to operation of the brake pedal BP, on the basis of the value detected by the stroke sensor BS and the pressure sensor Smc. That is, the electric current fed to the first and second linear solenoid valves SV 1  and SV 2  is controlled respectively, so that the wheel cylinder pressure detected by the pressure sensor Swc equals to a desired wheel cylinder pressure. Consequently, the hydraulic pressure controlled by the pressure control device PC in response to operation of the brake pedal BP is supplied to the wheel brake cylinder WC.  
      In the case where the pressure control device PC is normal as described above, according to the master cylinder MC, the master piston MP is not advanced substantially from such a position that the communication between the master pressure chamber C 1  and the atmospheric pressure chamber C 2  is blocked. Therefore, the simulator chamber C 4  is communicated with the atmospheric pressure chamber C 2  and finally with the reservoir RS, through the clearance CL between the seal member S 2  and the annular groove G 2  formed in the housing HS, so that the simulator chamber C 4  is under the atmospheric pressure. Accordingly, if the braking operation force applied to the simulator piston SP becomes equal to or greater than a compressive force for mounting the compression spring E 2  in the stroke simulator SM, the compression spring E 2  is compressed to provide the stroke of the simulator piston SP in response to the braking operation force. As a result, the stroke of the brake pedal BP is provided in response to the braking operation force.  
      On the contrary, in the case where the pressure control device PC including the pressure source PG and the like comes to be abnormal, the switching valve NO is de-energized (turned off) to be placed in its open position, so that the master cylinder MC and the wheel brake cylinder WC are communicated with each other, as shown in  FIG. 2 . At the same time, the first and second linear solenoid valves SV 1  and SV 2  are de-energized (turned off) to be placed in their closed positions, respectively, so that the hydraulic pressure is not supplied from the pressure source PG to the wheel brake cylinder WC. In this state, therefore, when the brake pedal BP is depressed, to advance the master piston MP by the second stroke or more from the initial position in response to operation of the brake pedal BP, the seal member S 2  will contact the large diameter bore B 3  formed in the housing HS, to block the communication between the simulator chamber C 4  and the atmospheric pressure chamber C 2 . Hereafter, therefore, the master piston MP will be advanced, without the compression spring E 2  being compressed in response to operation of the brake pedal BP, to discharge the hydraulic pressure from the master pressure chamber C 1  to the wheel brake cylinder WC.  
      In this case, even in such a state that the communication between the simulator chamber C 4  and the atmospheric pressure chamber C 2  is blocked, with the master piston MP being advanced, if the pressure control device PC comes to be abnormal during the operation of the brake pedal BP, i.e., when the stroke simulator SM is being stroked, the stroke simulator SM will be immediately retracted to its initial position by releasing the brake pedal BP to communicate the simulator chamber C 4  with the atmospheric pressure chamber C 2  through the seal member S 2  with its function as a check valve. In other words, the position of the simulator piston SP relative to the position of the master piston MP is placed to be in its initial position. Therefore, a so-called dead stroke could be prevented effectively, even if the brake pedal BP was operated more. Also, even if the brake pedal BP was rapidly released from such a state that the communication between the simulator chamber C 4  and the atmospheric pressure chamber C 2  was blocked, the simulator piston SP could only be moved rearward up to the position where it would contact the C-ring CR. In other words, as the retracting operation of the simulator piston SP is restricted by the C-ring CR at the rearmost position of the simulator piston SP to be determined relative to the master piston MP when the brake pedal BP has not been depressed, the master piston MP will not be prevented from being moved rearward. Therefore, the master piston MP could be moved rearward until its rear end will contact the stopper NH, so that the master pressure chamber Cl could be definitely opened to communicate with the reservoir RS.  
      Next, another embodiment of the present invention is explained referring to  FIG. 3 , wherein structural elements equivalent to those as shown in  FIG. 1  are designated by corresponding reference numerals. According to the present embodiment, the master piston MP as shown in  FIG. 1  is divided into two sections of a master piston MP 1  and an auxiliary piston MP 2 , the latter of which is formed into a stepped piston having a small diameter portion M 2 S and a large diameter portion M 2 L. A housing HS 2  is not formed with an annular groove corresponding to the annular groove G 2  as shown in  FIG. 1 , but formed with annular grooves for holding annular seal members S 5  and S 6  having cup-like cross sections, which act as the seal members S 2  and S 3  as shown in  FIG. 1 . According to the embodiment as shown in  FIG. 3 , therefore, the communication control device of the present invention is constituted by the seal member S 5  and a step formed between the small diameter portion M 2 S and the large diameter portion M 2 L. In other words, the communication control device includes the seal member S 5  disposed on the inner surface of the large diameter bore B 3 , and includes the small diameter portion M 2 S and the large diameter portion M 2 L. When the master piston MP 1  and auxiliary piston MP 2  are placed in their initial positions, respectively, as shown in  FIG. 3 , the simulator chamber C 4  is communicated with the atmospheric pressure chamber C 2  through the clearance CL defined between the seal member S 5  and the small diameter portion M 2 S. And, if the master piston MP 1  and auxiliary piston MP 2  are advanced from the initial positions by the second stroke or more, the large diameter portion M 2 L will contact the seal member S 5 , so that the communication between the simulator chamber C 4  and the atmospheric pressure chamber C 2  will be blocked.  
       FIG. 4  illustrates a further embodiment of the present invention, wherein structural elements equivalent to those as shown in  FIG. 1  are designated by corresponding reference numerals. According to the present embodiment, a master piston MP 3  is formed into a stepped piston having a small diameter portion M 3 S and a large diameter portion M 3 L. Like the embodiment as shown in  FIG. 3 , the housing HS 2  is not formed with an annular groove corresponding to the annular groove G 2  as shown in  FIG. 1 , but formed with the annular grooves for holding the annular seal members S 5  and S 6  having cup-like cross sections, which act as the seal members S 2  and S 3  as shown in  FIG. 1 . According to the present embodiment, however, the seal member S 5  is placed to be in contact with the large diameter portion M 3 L as shown in  FIG. 4 , when the master piston MP 3  is placed in its initial position, which is different from the embodiment as shown in  FIG. 3 . And, a communication hole CP is formed to penetrate the master piston MP 3  in a radial direction thereof, to communicate the annular chamber C 3  formed between the seal members S 5  and S 6  with the atmospheric pressure chamber C 2  formed between the seal member S 1  and the seal member S 5 .  
      Therefore, when the master piston MP 3  is placed in its initial position as shown in  FIG. 4 , the simulator chamber C 4  is communicated with the atmospheric pressure chamber C 2  through the communication hole CP. And, if the master piston MP 3  is advanced from the initial position by a predetermined distance or more, the communication hole CP will be closed by the seal member S 5  not to communicate with the annular chamber C 3  and the simulator chamber C 4 , so that the communication between the simulator chamber C 4  and the atmospheric pressure chamber C 2  will be blocked. Thus, the communication control device of the present invention is constituted by the communication hole CP and the seal member S 5 , according to the embodiment as shown in  FIG. 4 .  
       FIG. 5  illustrates a yet further embodiment of the present invention, wherein structural elements equivalent to those as shown in  FIG. 4  are designated by corresponding reference numerals. According to the present embodiment, a master piston MP 4  is formed into a stepped piston having a small diameter portion M 4 S and a large diameter portion M 4 L, like the master piston MP 3  as shown in  FIG. 4 . The housing HS 2  is not formed with the annular groove corresponding to the annular groove G 2  as shown in  FIG. 1 , but formed with the annular grooves for holding the annular seal members S 5  and S 6  having cup-like cross sections, which act as the seal members S 2  and S 3  as shown in  FIG. 1 . According to the present embodiment, communication grooves CG are formed longitudinally on the tip end portion of the large diameter portion M 4 L, as shown in  FIGS. 5 and 6 . When the master piston MP 4  is placed in its initial position as shown in  FIG. 5 , the seal member S 5  is positioned to contact the outer peripheral surface of the large diameter portion M 4 L, so that the simulator chamber C 4  communicates with the annular chamber C 3  and the atmospheric pressure chamber C 2 , through the communication grooves CG. And, if the master piston MP 4  is advanced from its initial position by a predetermined distance or more, the communication grooves CG will be closed by the seal member S 5  not to communicate with the annular chamber C 3  and the simulator chamber C 4 , so that the communication between the simulator chamber C 4  and the atmospheric pressure chamber C 2  will be blocked. Thus, the communication control device of the present invention is constituted by the communication grooves CG and the seal member S 5 , according to the embodiment as shown in  FIG. 5 .  
      Instead of the communication grooves CG as shown in  FIG. 6 , cut-out sections CT may be formed as shown in  FIG. 7 , around a part of the outer peripheral surface (as indicated by two-dotted chain line in  FIG. 7 ) of an end portion of the large diameter portion M 4 L to be cut out longitudinally. According to the present embodiment, therefore, when the master piston MP 4  is placed in its initial position, the simulator chamber C 4  is communicated with the annular chamber C 3  and the atmospheric pressure chamber C 2 , through the cut-out sections CT. And, if the master the master piston MP 4  is advanced from the initial position thereof by a predetermined distance or more, the cut-out sections CT will be closed by the seal member S 5  not to communicate with the annular chamber C 3  and the simulator chamber C 4 , so that the communication between the simulator chamber C 4  and the atmospheric pressure chamber C 2  will be blocked. Thus, the communication control device of the present invention is constituted by the cut-out sections CT and the seal member S 5 , according to the embodiment as shown in  FIG. 7 . According to the embodiments as described above, the master cylinder MC may be formed to provide a tandem master cylinder having a couple of master pressure chambers.  
      Next, referring to  FIG. 8 , explained is a yet further embodiment of the present invention, wherein structural elements equivalent to those describe in  FIG. 1  are designated by corresponding reference numerals. Instead of the master piston MP as shown in  FIG. 1 , a master piston MP 1  and an auxiliary piston MP 5  are accommodated in the housing HS according to the present embodiment. In a rear end portion of the auxiliary piston MP 5 , there are defined a small diameter bore M 2  and a large diameter bore M 3  for receiving therein the compression spring E 2  and simulator piston SP. According to the same manner as in the aforementioned embodiment, the C-ring CR is fitted into the annular groove MG of the auxiliary piston MP 2  in the state that the piston portion SP 1  of the simulator piston SP is received in the large diameter bore M 3 , to prevent the simulator piston SP from being moved rearward against the biasing force of the compression spring E 2 , so that the rearmost position of the simulator piston SP relative to the auxiliary piston MP 5  is defined. With the master piston MP 1  and auxiliary piston MP 5  accommodated in the small diameter bore B 2  and large diameter bore B 3  of the housing HS, the nut-like stopper NH is screwed into the open end portion B 4 . By means of the stopper NH, therefore, the master piston MP 1  and auxiliary piston MP 5  are prevented from being moved rearward against the biasing force of the compression spring El, to set the initial position as described later. For this purpose, after the initial position was adjusted, there has been remained a clearance (g) between the front end of the open end portion B 4  of the housing HS and the stopper NH, as shown in  FIG. 8 .  
      Then, in the same fashion as the embodiment as shown in  FIG. 1 , the communication control device according to the present embodiment is constituted such that if the master piston MP 1  and auxiliary piston MP 5  are advanced from their initial positions by the second stroke or more, the communication between the annular chamber C 3  (then, the simulator chamber C 4 ) and the atmospheric pressure chamber C 2  will be blocked by the seal member S 2  and the inner surface of the large diameter bore B 3 . The communication control device may be constituted as follows. At the outset, there are provided in advance several kinds of piston members having different dimensions (dx) from the tip end surface to the groove for receiving therein the front seal member S 2 . It is preferable to form an protrusion on the tip end of the auxiliary piston MP 5 , and adjust its height to set the dimension (dx). Then, an appropriate piston member to be used for the auxiliary piston MP 5  is selected from the several kinds of piston members prepared in advance, according to a dimension (d 0 ) from a rear end of the groove of the housing HS for receiving therein the seal member S 1  up to the annular groove G 2 , and a dimension (d 2 ) from the port P 3  of the master piston MP 1  to the rear end surface, which will contact the auxiliary piston MP 5 . In other words, the piston member is selected to provide the second stroke (dy), which is the distance of the auxiliary piston MP 5  advanced from the initial position thereof until it blocks the communication between the simulator chamber C 4  and the atmospheric pressure chamber C 2 , to be greater than the first stroke (d 1 ), which is the distance of the master piston MP 1  advanced from the initial position thereof until it blocks the communication between the master pressure chamber C 1  and the atmospheric pressure chamber C 2 , by a predetermined distance (k), to be (dy−d 1 =k), i.e., the piston member to meet (d 2 +dx−d 0 =k) is selected.  
      Or, there may be provided in advance a standard piston member (not shown) as an auxiliary piston without the simulator piston SP being assembled. Then, with air being supplied from the port P 2  of the housing HS connected to the reservoir RS, the moving distance (dy−d 1 ) of the standard piston and the master piston MP 1  is measured, when they are advanced from their initial positions until the communication between the master pressure chamber C 1  and the atmospheric pressure chamber C 2  is blocked, then the communication between the simulator chamber C 4  and the atmospheric pressure chamber C 2  is blocked. Then, the piston member may be selected to be served as the auxiliary piston MP 5 , on the basis of the measured result (dy−d 1 ). Or, with air being supplied from the port P 2  of the housing HS, the stopper NH is advanced to a position where the communication between the master pressure chamber C 1  and the atmospheric pressure chamber C 2  is blocked, then the stopper NH is slightly moved rearward from that position, whereby a port idle can be set to provide the first stroke (d 1 ).  
      The master cylinder with the braking stroke simulator as shown in  FIG. 8  is provided to constitute the hydraulic brake apparatus as shown in  FIG. 16 , which is substantially the same as the apparatus as shown in  FIG. 2 , except for the structure of the master cylinder with the braking stroke simulator as shown in  FIG. 1 . And, the master cylinder as shown in  FIG. 8  operates substantially in the same manner as the master cylinder as shown in  FIG. 1 , so that explanation of its basic operation is omitted herein. According to the present embodiment, even if the brake pedal BP was rapidly released from the state that the communication between the simulator chamber C 4  and the atmospheric pressure chamber C 2  was blocked, the simulator piston SP could only move up to the position where it would contact the C-ring CR. In other words, as the retracting operation of the simulator piston SP is restricted by the C-ring CR at its rearmost position of the simulator piston CP to be determined relative to the auxiliary piston MP 5  when the brake pedal BP has not been depressed, the auxiliary piston MP 5  will not be prevented from being moved rearward. Therefore, the master piston MP 1  and auxiliary piston MP 5  could be moved rearward until the rear end of the auxiliary piston MP 5  will contact the stopper NH, so that the master pressure chamber C 1  could be definitely opened to communicate with the reservoir RS.  
       FIGS.9-12  show embodiments of the cut-off stroke setting device according to the present invention for adjusting the distance between the master piston MP 1  and the auxiliary piston MP 5  to set the predetermined distance (dy−d 1 =k), an example of which is a rod disposed between the master piston MP 1  and the auxiliary piston MP 5 . According to the embodiment as shown in  FIG. 9 , it is so constituted that an appropriate rod RD is selected from several kinds of rods of different dimensions. The rod RD is disposed between the recess M 0  formed on the rear end surface of the master piston MP 1  and the recess M 4  formed on the front end surface of the auxiliary piston MP 5 , as shown in  FIG. 9 .  
       FIG. 10  shows another embodiment of the cut-off stroke setting device according to the present invention for adjusting the distance between the master piston MP 1  and the auxiliary piston MP 5  to be fixed at the predetermined distance. As shown in  FIG. 10 , an adjusting rod A 1  is screwed into a cylindrical protrusion M 5 , which is formed to extend into the recess M 0  of the master piston MP 1  from the front end of the auxiliary piston MP 5 , so that the tip end of the adjusting rod A 1  contacts the inner surface of the recess M 0  of the master piston MP 1 . According to the embodiment as shown in  FIG. 10 , therefore, the distance between the master piston MP 1  and the auxiliary piston MP 5  can be adjusted into an appropriate value, by adjusting the depth of the adjusting rod A 1  screwed into the protrusion M 5 . In order to ensure the fixed state of the stopper NH to the open end portion B 4  of the housing HS, and ensure the fixed state of the adjusting rod A 1  to the auxiliary piston MP 5 , as shown in  FIG. 15  for example, it is preferable to set the diameter Rtm of the bottom of thread of a male screw Tm (at the sides of the stopper NH and adjusting rod A 1 ) to be slightly larger than the inner diameter Rtf of a female screw Tf (at the sides of the housing HS and auxiliary piston MP 5 ), and to screw it firmly so that the parts with the male screw Tm being screwed would not be rotated, whereby positioning of the parts can be made surely.  
      FIGS. 11  and  12  show a further embodiment of the cut-off stroke setting device for adjusting the distance between the master piston MP 1  and the auxiliary piston MP 5  to be fixed at the predetermined distance. In contrast to the device as shown in  FIG. 10 , wherein the adjusting rod A 1  is screwed into the cylindrical protrusion M 5 , an adjusting rod A 2  as shown in  FIG. 11  is pressed into the cylindrical protrusion M 5  to be fixedly held therein. And, an adjusting rod A 3  as shown in  FIG. 12  is received in a cylindrical protrusion M 6 , which is deformed, or caulked to hold the adjusting rod A 3  fixedly.  
      As for the port idle setting device, a stopper may be provided for adjusting the initial position of at least one of the master piston MP 1  and the auxiliary piston MP 5 , to be secured to the housing HS. According to the embodiments as shown in  FIGS.8-12 , it is so constituted that the stopper NH is screwed into the open end portion B 4  of the housing HS with threaded grooves formed on the inner side of the open end portion B 4 . Then, with air being supplied from the port P 2  of the housing HS, the stopper NH is advanced from the initial position thereof until the communication between the master pressure chamber C 1  and the atmospheric pressure chamber C 2  is blocked, and then the stopper NH is slightly moved rearward, and fixed to set the port idle to provide the distance (d 1 ). In order to fix the stopper, a stopper NL as shown in  FIG. 13  may be deformed in an open end portion B 6  of the housing HS, to form a caulked portion CK as shown in  FIG. 13 . Or, a stopper NK as shown in  FIG. 14  may be pressed into an open end portion B 5  of the housing HS. In the latter case, a standard stopper member (not shown) with a slightly small outer diameter not to be pressed into the open end portion B 5 , is prepared in advance, and a longitudinal position of the stopper is set as described above. Then, the stopper NK is pressed into the open end portion B 5  up to the longitudinal position, so that the port idle may be set to provide the distance (d 1 ).  
      It should be apparent to one skilled in the art that the above-described embodiments are merely illustrative of but one of the many possible specific embodiments of the present invention. Numerous and various other arrangements can be readily devised by those skilled in the art without departing from the spirit and scope of the invention as defined in the following claims.