Abstract:
The object of the present invention is to provide a brake fluid pressure generating apparatus which has a master cylinder with supplemental pressure room which operates the master cylinder independently of a depression of a brake pedal without structure complication. To perform this object, a master cylinder piston slides over seal cups which are arranged on a master cylinder side.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to a brake fluid pressure generating apparatus. More particularly, the apparatus is applied to a mobile that is equipped with the brake fluid circuit. 
     BACKGROUND OF THE INVENTION 
     In general, a vehicle brake system includes a brake booster and master cylinder to deliver a pressurized brake fluid to each wheel cylinder. The brake booster boosts the depression force to a brake pedal and outputs boosted force which is response to the depression of a brake pedal. The master cylinder converts the boosted force into a brake fluid pressure. 
     The engine vacuum tends to drop in the latest vehicle compared with the previous vehicle, on account of this reason a sufficient and/or stable boosted force is not generated utilizing a brake booster. To overcome the above-mentioned disadvantage, a pressure device which is used for traction control system is utilized for the brake system. In other words, a brake fluid in a reservoir tank is pressurized using the pressure device and then the pressurized brake fluid is supplied straight to each wheel cylinder. 
     The brake fluid amount in a master cylinder is increased by using supplemental pressure device and an excess amount of the brake fluid is existing in the brake fluid circuit. Therefore, when the brake pedal is released, the excesses amount of the brake fluid flows back to the master cylinder. The excess brake fluid operates a master cylinder piston from the initial position to unusual position. This movement damages seal materials in the master cylinder in which produces a fluid tight manner. Consequently, when the supplemental pressure device is utilized in the brake fluid circuit, a solid designed master cylinder seal structure is required. This will be a cause of a cost increase for the device. 
     A third pressure room is applied to the master cylinder to avoid supplemental pressure device disadvantage. In this construction, a brake fluid is supplied to the brake fluid independent of the primary brake fluid circuit. However, providing the third pressure room in the master cylinder complicates the structure of the master cylinder. Further, the third pressure room lengthens the size of the master cylinder. 
     The object of the present invention is to provide a brake fluid pressure generating apparatus which has a third pressure room with a simple structure and shortens the length of the master cylinder. 
     SUMMARY OF THE INVENTION 
     To achieve the subject matter of the present invention, a brake fluid pressure generating device composed of a brake booster which boosts the depression force to a brake pedal, a master cylinder that generates brake fluid pressure in response to the depression of the brake pedal using a master cylinder piston, a pressurized media introduction room defined between the brake booster and a master cylinder piston that operates the master cylinder independently of the brake booster and the master cylinder piston slides over a plurality of seal cups that are arranged on the master cylinder. 
     According to the invention, a brake fluid pressure regulation valve which is disposed between a reservoir tank and the pressurised media introduction room regulates the brake fluid d pressure from the pressurised media introduction room to the reservoir tank. This arrangement provides an appropriate brake fluid pressure to the wheel cylinders independently of the depressing of the brake pedal. 
     According to a further feature of the invention, the brake fluid pressure regulation valve has a cut off function that isolates the pressurised media introduction room from said reservoir tank. This specific arrangement provides substitutive function for a parking brake. 
     According to a further feature of the invention, the master cylinder piston is divided into two pieces and a cross section of the first master cylinder piston is relatively bigger than a cross section of the second master cylinder piston. This construction provides large pressurized brake fluid receiving section on the first master cylinder piston and minimizes a brake fluid pressure in the pressurised media introduction room to slide the first master cylinder piston. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     Additional features and characteristics of the present invention will become more apparent from the following detailed description considered with reference to the accompanying drawing figures in which like elements are designated by like reference numerals and wherein: 
     FIG. 1 is an fluid pressure circuit of the brake fluid pressure generating apparatus according to the invention; and 
     FIG. 2 is a cross sectional view of the master cylinder according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     This invention will be described in according to preferred embodiment which is shown in attached drawings. FIG. 1 shows a fluid circuit which contains the brake fluid pressure generating apparatus. A brake booster I has an input rod  2  and an output rod  3 . A brake pedal  4  is linked to the input rod  2 . A depression force, which is applied to the brake pedal  4 , is transmitted to the output rod  3  by way of the input rod  2 . In detail, when the brake pedal  4  is depressed, a pressure difference between a constant pressure chamber  5  and a variable pressure chamber  6  occurs in a brake booster  1  and in response to this pressure difference, an output force, which is generated in proportion to a brake pedal  4  depression force, is applied to the output rod  3 . The output force from the output rod  3  is transmitted to the master cylinder  10 . In this embodiment, the master cylinder  10  is a tandem type master cylinder. 
     The master cylinder  10  has a first master cylinder piston  11  and a second master cylinder piston  12  and three pressure rooms  13 ,  14  and  15  are formed by these pistons  11  and  12 . The first pressure room  13  is formed between the first master cylinder piston  11  and the second master cylinder piston  12 . The second pressure room  14  is formed between the second master cylinder piston  12  and an end wall  16  of master cylinder  10 . The third pressure room  15  is formed between the first master cylinder piston  11  and a front end wall  17  of the master cylinder  10 . In the first pressure room  13 , a first spring  18  is disposed between the first master cylinder piston  11  and the second master cylinder piston  12 . In the second pressure room  14 , a second spring  19  is disposed between the first master cylinder piston  11  and the second master cylinder piston  12 . In this embodiment, the first master cylinder piston  11  is used as master cylinder piston and the third pressure room  15  has a function as a pressurised media introduction room. 
     The master cylinder  10  has a first port  21 , a second port  22 , a third port  23  and a fourth port  24 . The first and second ports  21  and  22  are communicated with the first pressure room  13 . The third and fourth ports  23  and  24  are communicated with the second pressure room  14 . The first and third ports  21  and  23  communicate with a reservoir tank  27  by way of fluid paths  25  and  26 , respectively. 
     Therefore, the brake fluid, which is stored in the reservoir tank  27 , is supplied to the first pressure room  13  through the fluid path  25  and the first port  21 . Further, the brake fluid is supplied to the second pressure room  14  through the fluid path  26  and the third port  23 . 
     The second port  22  which communicates with the first pressure room  13  is communicated with a not shown wheel cylinder through a fluid path  28 . The fourth port  24  which communicates with the first pressure room  14  is communicated with the not shown wheel cylinder through a fluid path  29 . 
     A front side (the brake booster  1  side) of the first master cylinder piston  11  is operatively connected to the output rod  3 . In response to depression of the brake pedal  4 , the first master cylinder piston  11  moves toward a rear side (opposite side of the brake booster  1  side) through the input rod  2  and the output rod  3 . In response to this first master cylinder piston  11  movement, a communication between the first pressure room  13  and the first port  21  is controlled. In this condition, when the first master cylinder piston  11  further moves toward the rear side, the brake fluid in the first pressure room  13  is pressurized and the pressurized brake fluid is supplied to the wheel cylinder through the second port  22  and the fluid path  28 . 
     When the brake fluid pressure in the first pressure chamber  13  is increased, the second master cylinder piston  12  moves toward a rear side. In response to this second master cylinder piston  12  movement, a communication between the second pressure room  14  and the second port  22  is controlled. In this condition, when the second master cylinder piston  12  further moves toward the rear side, the brake fluid in the second pressure room  14  is pressurized and the pressurized brake fluid is supplied to wheel cylinder through the fourth port  24  and the fluid path  29 . 
     When the brake pedal  4  is released, the first and second pistons  1   1  and  12  return to initial position using fluid pressure, and a compression force of a first and second springs  18  and  19 . Therefore, the first and third ports  21  and  23  communicate with the first pressure room  13  and the second pressure room  14 , respectively. The fluid pressure, which is applied to the wheel cylinders, is released. 
     A fifth port  30 , which communicates with the third pressure room  15 , is formed in the master cylinder  10 . The fifth port  30  communicates with the reservoir tank  27  through a first one-way valve  31 , a fluid pump  32  and a second one-way valve  33 . The fluid pump  32  is driven by an electric motor  34  and generates fluid pressure. The brake fluid in the reservoir tank  27  is applied to the third pressure room  15  through the second one-way valve  33 , the fluid pump  32  and the first one-way valve  31 . When a pressurized brake fluid is applied to the third pressure room  15 , the first master cylinder piston  11  moves toward the rear side. In this construction, the first master cylinder piston  11  is independently controlled with the operation of the brake booster  1 . 
     The fifth port  30  also communicates with the reservoir tank  27  through an electronic valve  35  and a third one-way valve  36  is connected to the electronic valve  35  in parallel. The electronic valve  35  has three stage functions “a”, “b” and “c” and functions as a brake fluid pressure regulation valve. When the electronic valve is positioned at “a”, a communication between the third pressure room  15  and the reservoir tank  27  is cut off. When the electronic valve is positioned at “b”, the third pressure room  15  and the reservoir tank  27  communicate with each other. When the electronic valve is positioned at “c”, a communication between the third pressure room  30  and the reservoir tank  27  is regulated and brake fluid flow from the third pressure room  15  to the reservoir  27  is permitted. 
     A positioning of the electronic valve  35  and the electric motor  34  are controlled by a controller  40 . A brake pedal sensor  41 , a vacuum pressure sensor  42  and a fluid pressure sensor  43  are connected to the controller  40 . The brake pedal sensor  41  detects a stroke of depression of the brake pedal  4  or detects the amount of the depression force of the brake pedal  4 . In this embodiment, when the brake pedal  4  is depressed, the controller  40  drives the electric motor  34  and puts the electronic valve into the “a” or “c” position. When the brake pedal  4  is released, the controller  40  turns the electric motor  34  off and controls the electronic valve  35  in the “b” or “c” position in response to the actual returning condition of the brake pedal  4 . 
     The vacuum pressure sensor  42  detects actual pressure in the constant pressure chamber  5  and the fluid pressure sensor  43  detects actual fluid pressure in the fluid path  28 . In response to the signals from the sensors  42  and  43 , the controller  40  computes the amount and the pressure of the brake fluid to the third pressure room  15 . The electric motor  34  is operated in response to the sensors  42  and  43  using predetermined control program. 
     The details of the construction of the master cylinder  10  are described with reference to FIG.  2 . In FIG. 2, a main body  51  of the master cylinder  10  has a cylindrical shape and has an opening and bottom wall. A cover  52  is screwed and is fixed to the opening of the main body  51 . The brake booster  1  is mounted on the cover  52 . A cylindrical first sleeve  53  and a cylindrical second sleeve  54  are disposed in the inner space which is formed by the main body and the cover  52 . 
     The first sleeve  53 , which forms the pressurised media introduction room (the third pressure room  15 ), has a large diameter portion  53   a , a medium diameter portion  53   b  and a small diameter portion  53   c . The large diameter portion  53   a  is supported by the inner wall of the main body  51 . The medium diameter portion  53   b  and the small diameter portion  53   c  are supported by the inner wall of the cover  52 . A continuous groove  55  is formed at the outer surface of the large diameter portion  53   a  of the first sleeve  53 . The fifth port  30  is formed on the opposite side of the main body  51 . 
     The first sleeve  53  has a first inner room  56  and a second inner room  57  which have different diameters from each other. The first inner room  56  is formed at the large diameter portion  53   a  and a medium diameter portion  53   b  and a large first master cylinder piston  11   a  is arranged in the first inner room  56 . A fluid seal cup  58  is arranged at a step shaped portion of the large diameter portion  53   a . A fluid tight manner between the large first master cylinder piston  11   a  and the first sleeve  53  is established. 
     The second inner room  57  is smaller than the first inner room  56  at its diameter. The second inner room  57  is formed at the small diameter portion  53   c  and a small first master cylinder piston  11   b  is supported at the second inner room  57 . Therefore, the small first master cylinder piston  11   b  slides over the first sleeve  53 . The diameter of the first master cylinder piston  11   a  is relatively bigger than the second master cylinder piston  11   b . In this embodiment, the first master cylinder piston  11  is composed of the large first master cylinder piston  11   a  and the small first master cylinder piston  11   b  and each piston  11   a  and  11   b  can move independently. A continuous groove is formed on the inner surface of the second inner room  57 , and a seal cup  58   a  is disposed in the groove. A fluid seal cup  58   b  is also arranged at a step shaped portion, which is formed at the end of the first sleeve  53 , of the small diameter portion  53   c . A fluid tight manner between the small first master cylinder piston  11   b  and the first sleeve  53  is established by seal cups  58   a  and  58   b.    
     One end of the small first master cylinder piston  11   b  is connected to the output rod  3  through an opening  52   a  which is formed at the end of the cover  52 . A flange  59  is formed at the other end of the small first master cylinder piston  11   b . The flange  59  is positioned in the first inner room  56  and the diameter of the flange  59  is bigger than diameter of the second inner room  57 . However, the diameter of the flange  59  is smaller than diameter of the first inner room  56 . The flange  59  of the small first master cylinder piston  11   b  contacts with the large first master cylinder piston  11   a.    
     The third pressure room  15  is formed by end wall of the large first master cylinder piston  11   a , an outer surface of the flange  59 , an inner surface of the first inner room  56  and the front end wall  17 . A diameter of the first inner room  56  is a little larger than the diameter of the large first master cylinder piston  11   a . Therefore, a clearance is defined between the first sleeve  53  and the large first master cylinder piston  11   a . A first fluid path  60  is formed between the continuous groove  55  and the first inner room  56 . Therefore, the third pressure room  15  communicates with the fifth port  30  through the clearance between the first sleeve  53  and the large first master cylinder piston  11   a , the first fluid path  60  and the continuous groove  55 . 
     In accordance with the movement of the output rod  3  of the brake booster  1 , the large first master cylinder piston  11   a  slides toward the rear side together with the small first master cylinder piston  11   b . In this condition, if the fluid pressure is introduced from the fifth port  30 , the large first master cylinder piston  11   a  slides toward the rear side utilizing the fluid pressure in the third pressure room  15 . 
     Without movement of the output rod  3  of the brake booster  1 , for instance, if the fluid pressure is introduced from the fifth port  30 , the large first master cylinder piston  11   a  slides toward the rear end only using the fluid pressure which is introduced in the third pressure room  15 . In this condition, the fluid pressure in the third pressure room  15  is applied to a cross section of the large first master cylinder piston  11   a . When the large first master cylinder piston  11   a  is apart from the small first master cylinder piston  11   b  and the small first master cylinder piston  11   b  slides in response to the movement of the output rod  3 , the fluid pressure in the third pressure room  15  is increased. Therefore, the large first master cylinder piston  11   a  further slides to the rear end. 
     The second sleeve  54  has a large diameter portion  54   a , a medium diameter portion  54   b  and a small diameter portion  54   c  and the second sleeve  54  is disposed in the main body  51 . The large first master cylinder piston  11   a  and the second master cylinder piston  12  are arranged in the inner surface of the second sleeve  54 . A space that is defined between the large first master cylinder piston  1 la and the second master cylinder piston  12  is the first pressure room  13 . 
     A first groove  61  is formed on the inner surface of the second sleeve  54 . The first groove  61  is formed along the center axis of the second groove  54 . A second fluid path  62  is formed on the small diameter portion  54   c  of the second sleeve  54  and an outer surface of the second sleeve  54  communicates with the first groove  61  through the second fluid path  62 . The second fluid path  62  communicates with the second port  22 . Therefore, the first pressure room  13  communicates with the second port  22  through the first groove  61  and the second fluid path  62 . 
     A step portion is formed at the each end of the second sleeve  54 , and a seal cup  63  and  64  are arranged on the step portion. A first guide member  65  is arranged between the first sleeve  53  and the second sleeve  54 . The first guide member  65  is cylindrical and the large first master cylinder piston  11   a  is slidably arranged on the inner surface of the first guide member  65 . Both ends of the first guide member  65  have a relatively small diameter compared with the middle portion of the first guide member  65  and either end of the first guide member  65  is close to the seal cups  58  and  63 . A fluid tight manner between the large first master cylinder piston  11   a  and the first sleeve  53 , a fluid tight manner between the large first master cylinder piston  11   a  and the second sleeve  53  are established utilizing the seal cup  58  and  63 . A third fluid path  66  is formed between an inner surface and an outer surface of the first guide member  65 . The third fluid path  66  communicates with the first port  21  through the clearance which is defined between first and second sleeve  53  and  54 . 
     A concave portion  67  is formed at the rear side of the main body  51  and the concave portion  67  accepts the second master cylinder piston  12 . A step portion is formed at the opening portion of the concave portion  67 , and a seal cup  68  is disposed on the step portion. The second pressure room  14  is defined between the concave portion  67  and the second master cylinder piston  12 . A second groove  69  is formed on the inner surface of the concave portion  69 . The second groove  69  is formed along the center axis of the main body  51 . The second groove  69  communicates with the fourth port  24 . Therefore, the second pressure room  14  communicates with the fourth port  24  through the second groove  69 . 
     A second guide member  70  is arranged between the second sleeve  54  and the concave portion  69 . The second guide member  70  is cylindrical and the second master cylinder piston  12  is slidably arranged on the inner surface of the second guide member  70 . Both ends of the second guide member  70  have a relatively small diameter compared with the middle portion of the second guide member  70  and either end of the second guide member  70  is close to the seal cups  64  and  68 . A fluid tight manner between the second master cylinder piston  12  and the second sleeve  54 , a fluid tight manner between the second master cylinder piston  12  and the main body are established utilizing the seal cup  64  and  68 . A fourth fluid path  71  is formed between an inner surface and an outer surface of the second guide member  70 . The fourth fluid path  71  communicates with the third port  23 . 
     The large first master cylinder piston  11   a  has a concave portion  72  and a first communication port  73  is formed on the sidewall of the large first master cylinder piston  11   a . When the first large piston  1 I a is placed at an initial position (no operating condition), the first pressure room  13  communicates with the first port  21  through the concave portion  72  and the third fluid path  66 . When the large first master cylinder piston  11   a  slides into the rear side, the first communication port  73  slides into the left side of the seal cup  63 . Therefore, the first pressure room  13  discommunicates with the first port  21 . 
     A guide rod  74  is disposed and fixed at the center portion of the concave portion  72  and a receiving seat  75  engages with the guide rod  74 . The guide rod  74  has a flange  74   a . The flange  74   a  engages with the receiving seat  75  and regulates movement in the front side direction. The receiving seat  76  has a bending portion in the rear side and the first spring  18  is disposed between the bending portion of the receiving seat  75  and a bottom of the concave portion  72  with predetermined compression. The first spring  18  applies a compression force to the second master cylinder piston  12 . Therefore, if the first master cylinder piston  11  slides to the rear side, the second master cylinder piston  12  slides into rear side direction. 
     The second master cylinder piston  12  has a concave portion  76  and the second spring  19  is disposed between the concave portion  76  and the end wall  16  with predetermined compression. A second communication port  77  is formed on the sidewall of the second master cylinder piston  12 . When the second master cylinder piston  12  is placed at an initial position (no operating condition), the second pressure room  14  communicates with the third port  23  through the concave portion  76 , the second communication port  77  and the fourth fluid path  71 . When the second master cylinder piston  12  slides into the rear side, the second communication port  77  slides into the left side of the seal cup  68 . Therefore, the second pressure room  14  discommunicates with the third port  23 . 
     A union  78 , which connects to the reservoir tank  27 , is attached to the main body  51 . The union  78  has fluid paths  26  and  26 . The fluid path  25  communicates with the first port  21  and the fluid path  26  communicates with the third port  23 . 
     The following are features of the brake fluid pressure generating apparatus. 
     In this embodiment, the third pressure room  15  is formed in the master cylinder  10  and the brake fluid is supplied to the third pressure room  15  by the fluid pump  32 . The first master cylinder piston  11  (the large first master cylinder piston  11   a ) is driven when the brake fluid is supplied into the third pressure room  15  without depression of the brake pedal  4  and/or operation of the brake booster  1 . Therefore, even if the brake booster  1  is under a disabled and/or unstable condition, the master cylinder  10  generates stable operation by using the brake fluid which is supplied into the third pressure room  15  by the fluid pump  32 . 
     In this embodiment, the controller  40  controls the electric motor  34  and drives the fluid pump  32  in response to the depression of the brake pedal  4  which is detected by the brake pedal sensor  41 . When the brake pedal  4  is depressed, the controller  40  drives the electric motor  34  and puts the electronic valve into the “a” or “c” position. When the brake pedal  4  is released, the controller  40  turns the electric motor  34  off and controls the electronic valve  35  in the “b” or “c” position in response to actual returning condition of the brake pedal  4 . Therefore, the master cylinder  10  is accurately controlled in response to the depression of the brake pedal  4 . 
     In this embodiment, the master cylinder  10  is operated without depression of the brake pedal  4  by supplying the brake fluid into the third pressure room  15 . Namely, the master cylinder  10  is operated without the brake booster  1 . If the controller  40  computes and/or detects a driving condition accurately, the present brake fluid pressure generating apparatus is applicable to an automatic brake control system by using various driving conditions. 
     In this embodiment, when the electronic valve  35  is placed in the “a” position, the third pressure room  15  is disconnected from the reservoir tank  27 . When the brake fluid is supplied into the third pressure room  15  and then the electronic valve  35  is placed in the “a” position, the apparatus functions as a parking brake while the vehicle is parking. 
     In this embodiment, the brake fluid pressure in the third pressure room  15  is regulated when the electronic valve  35  is placed in the “c” position. An appropriate braking force is generated when the brake fluid is supplied into the third pressure room  15  under the vehicle parking. 
     In this embodiment, the brake fluid is supplied into a closed fluid circuit which is comprised of the third pressure room  15 , the fluid pump  32 , the electronic valve  35  and plurality of one-way valves  31 ,  33 ,  36 . Therefore, the brake fluid, which is supplied into the third pressure room  15 , does not come into a main brake fluid circuit. The main brake fluid circuit is connected to the wheel cylinders. Though the brake fluid is supplied into the third pressure room  15 , the amount of the brake fluid in the main brake fluid circuit is not increased. Consequently, it is not need to make strong the fluid tight manner of the master cylinder  10 . 
     In this embodiment, the first master cylinder piston  11  is divided into the large first master cylinder piston  11   a  and the small first master cylinder piston  11   b , the brake fluid is applied cross section of the large first master cylinder piston  11   a . Namely, the large first master cylinder piston  11   a  is operated by relatively low brake fluid pressure. As a result, relatively low capacity fluid pump  32  would apply this embodiment and no need to make the master cylinder  10  strong. 
     In this embodiment, the seal cups  58 ,  63 ,  58   a  and  58   b  are arranged on the first sleeve  53  and the second sleeve  63 , respectively. The fluid tight manner in the third pressure room  15  is established by the seal cups  58  and  58   a . The seal cup  58  establishes the fluid tight manner between the large first master cylinder piston  11   a  and the first sleeve  53 . The seal cup  63  establishes the fluid tight manner between the large first master cylinder piston  11   a  and the second sleeve  54 . The seal cups  64  and  68  are arranged on the second sleeve  54  and the concave portion  67  of the main body  51 , respectively. The seal cup  64  establishes the fluid tight manner between the second master cylinder piston  12  and the second sleeve  54 . The seal cup  68  establishes the fluid tight manner between the second master cylinder piston  12  and the main body  51 . Therefore, this seal cup arrangement shortens the length of the master cylinder  10  compared with a conventional type master cylinder. In the conventional type master cylinder, the seal cups are arranged on the pistons  11  and  12 . 
     Though the third pressure room  15  lengthen the size of the master cylinder  10  comparing with the conventional type master cylinder, the arrangement of the seal cups of this embodiment shortens the length of the master cylinder  10 . There is no seal cups on the first master cylinder piston  11 , so the first master cylinder piston  11  is placed close to the second master cylinder piston  12 . This placement enables the length of the master cylinder  10  to be shortened. 
     While the seal cups  58  and  58   a  are arranged on the first sleeve  53 , the fifth port  30  and the first fluid path  60  are designed at any position between the seal cup  58  and the seal cup  58   b . In this arrangement, the third pressure room  16  is able to be placed in the brake booster  1  and the length of the entire brake fluid pressure generation apparatus is shortened. 
     The embodiment of the present invention is not limited above mentioned embodiment and some modifications would be able to apply. 
     In this embodiment, the first piston is divided into two pieces, however the piston can be designed as a one piece body. In this modified embodiment, a reaction force which is applied to the output rod  3  into the front side through the small first master cylinder piston  11  b is not generated. This invention would be able to apply to the master cylinder which has a sole piston. A electronic controlled proportional valve applies to this embodiment for three stage electronic valve  35 . The first sleeve  53  and the cover  52  can be designed as one body. 
     In accordance with features of the present invention, the length and/or size of the master cylinder is shortened. Further, an appropriate braking force is generated without depression of the brake pedal. Further more, the first piston slides into rear side without using relatively high brake fluid pressure.