Abstract:
Reaction transmitting means of a brake booster includes a parallel combination of a first and a second reaction transmitting path each transmitting a brake reaction from a reaction disc to a valve plunger. The first reaction transmitting path includes a spring charged to a preset load, and the second reaction transmitting path includes a viscoelastic member as may be formed of rubber. With this construction, a hysteresis is obtained in both a low range of servo ratio during an initial phase of operation of a brake booster and a higher range of servo ratio during a later phase of operation, which occur before and after the spring in the reaction transmitting mechanism is compressed, thus allowing a good brake feeling to be maintained.

Description:
FIELD OF THE INVNETION 
     The invention relates to a brake booster, and more particularly, to a brake booster which is provided with reaction transmitting means including a spring. 
     DESCRIPTION OF THE PRIOR ART 
     A brake booster is known in the art which exhibits a reduced servo ratio during the initial phase of the braking operation of the brake booster and exhibits an increased servo ratio during a later phase of the braking operation. 
     In a brake booster of the kind described, reaction transmitting means is interposed between a reaction disc and a valve plunger, and comprises a first and a second retainer disposed to be displaceable relative to each other, a spring disposed between the first and the second retainer and charged to a preset load to maintain the both retainers spaced apart normally, and a stop member which prevents the first and the second retainer from being spaced apart beyond a given spacing (Japanese Lead-Open Patent Application No. 85,442/1996). 
     With this construction, when a brake pedal is depressed to actuate the brake booster, the reaction transmitting means merely transmits a brake reaction which is transmitted from the reaction disc to the valve plunger through the first retainer, the spring and the second retainer until the spring within the reaction transmitting means is compressed, whereby the prevailing servo ratio can be maintained relatively low. This improves the maneuverability in a range where a braking force of a reduced magnitude is required. 
     On the other hand, when the spring within the reaction transmitting means is compressed, the second retainer and the valve plunger will be advanced relative to the first retainer and the plate plunger by an amount corresponding to the compression of the spring, and accordingly, this is equivalent to a further depression of the brake pedal by an amount corresponding to the compression of the spring. The prevailing servo ratio can be chosen relatively high, thereby allowing a braking force of an increased magnitude to be obtained with a reduced force of depression. 
     Because the reaction disc comprises a viscoelastic member such as rubber, the brake booster incorporating the reaction disc exhibits a hysteresis, a phenomenon that the magnitude of an output from the brake booster becomes different upon depression and upon release of the brake pedal if the force with which the brake pedal is depressed is maintained same. 
     In the presence of the hysteresis, if a driver unconsciously decreases the force of depression slightly after he has ceased to increase the force with which the brake pedal is depressed, a braking force of the same magnitude is maintained, bringing forth an advantage that the brake feeling is improved. 
     However, in the brake booster which is provided with the reaction transmitting means, when the spring within the reaction transmitting means is compressed, a flow path switching action takes place within the valve mechanism of the brake booster as the spring changes its length, whereby the hysteresis caused by the reaction disc is accommodated by the spring, resulting in the magnitude of an output from the brake booster which remains substantially equal upon depression and upon release of the brake pedal. 
     As a consequence, if a driver unconsciously decreases the force of depression slightly after he has ceased to increase the force with which the brake pedal is depressed, the braking force will be immediately reduced, and a driver then feels as if the brake is ineffective, thus disadvantageously degrading the brake feeling. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, the invention provides a brake booster which is provided with reaction transmitting means including a spring, but which is capable of maintaining a brake feeling in good condition. 
     Thus, the invention relates to a brake booster including a valve body slidably disposed within a shell, a valve mechanism disposed within the valve body, an input shaft for driving a valve plunger, which forms part of the valve mechanism, back and forth to switch a flow path in the valve mechanism, an output shaft slidably mounted on the valve body, a reaction disc interposed between one end of the output shaft and the valve plunger, and reaction transmitting means interposed between the reaction disc and the valve plunger for transmitting a brake reaction from the reaction disc to the valve plunger. In accordance with the invention, the reaction transmitting means includes a first and a second reaction transmitting path disposed in parallel for transmitting a brake reaction from the reaction disc to the valve plunger, the first reaction transmitting path including a spring charged to a preset load so that the brake reaction from the reaction disc is transmitted through the spring to the valve plunger while the second reaction transmitting path includes a viscoelastic member so that the brake reaction from the reaction disc is transmitted to the valve plunger through the viscoelastic member. 
     With the described construction, when the pedal is depressed to actuate the brake booster, the servo ratio can be maintained relatively low in the similar manner as in the prior art until the spring within the reaction transmitting means becomes compressed. 
     At this time, since the valve plunger causes a compression of the reaction disc through the reaction transmitting means and moves forward, the reaction disc is effective to produce a hysteresis which provides an output from the brake booster of a differing magnitude upon depression and upon release of the brake pedal. 
     On the other hand, as the spring within the reaction transmitting means becomes compressed, the valve plunger will be advanced relative to the valve plunger by an amount corresponding to the compression of the spring, equivalently resulting in an effect that the brake pedal is further depressed by an amount corresponding to the compression of the spring, thus increasing the servo ratio. 
     At this time, as the spring within the reaction transmitting means is compressed, the brake reaction from the reaction disc is transmitted to the valve plunger through the spring in the first reaction transmitting path while simultaneously the brake reaction from the reaction disc is transmitted to the valve plunger through the viscoelastic member in the second reaction transmitting path. 
     When the brake reaction acts upon the viscoelastic member in the second reaction transmitting path, the viscoelastic member is compressed in accordance with the magnitude of the brake reaction, whereby the viscoelastic member is effective to provide the hysteresis in the similar manner as the reaction disc. 
     As a consequence, the hysteresis is provided during an initial phase of operation of the brake booster which is prior to the commencement of the compression of the spring and during which the servo ratio is in a low range and during a later phase which is after the commencement of compression of the spring and during which the servo ratio is in a greater range, thus enabling the brake feeling to be maintained in good condition. 
     Above and other objects and advantages of the invention will become apparent from the following description of several embodiments of the invention with reference to the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross section of a first embodiment of the invention; 
     FIG. 2 is a cross section, to an enlarged scale, of part shown in FIG. 1; 
     FIG. 3 is a cross section showing the operational condition of the part shown in FIG. 2; 
     FIG. 4 is a cross section of a second embodiment of the invention; 
     FIG. 5 is a cross section taken along the line V—V shown in FIG. 4; 
     FIG. 6 is a cross section of a third embodiment of the invention; 
     FIG. 7 is a cross section of a fourth embodiment of the invention; 
     FIG. 8 is a cross section of a fifth embodiment of the invention; 
     FIG. 9 is a cross section of a sixth embodiment of the invention; 
     FIG. 10 is a cross section of a seventh embodiment of the invention; 
     FIG. 11 is a cross section of an eighth embodiment of the invention; and 
     FIG. 12 is a cross section of a ninth embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Several embodiments of the invention will now be described. Referring to FIG. 1, a brake booster  1  of tandem type includes a shell  2 , the interior of which is divided by a centerplate  3  to define a front chamber  4  and a rear chamber  5  thereacross. A tubular valve body  6  slidably extends through the rear side of the shell  2  and the centerplate  3 , with seal means  7 ,  8  maintaining a hermetic seal thereat. 
     A front power piston  10  and a rear power piston  11  are connected to the outer periphery of the valve body  6  in regions which are located within the front chamber  4  and the rear chamber  5 , respectively, and a front diaphragm  12  and a rear diaphragm  13  are applied to back surfaces of the power pistons  10 ,  11 , respectively. In this manner, a constant pressure chamber A and a variable pressure chamber B are defined across the front diaphragm  12  within the front chamber  4  while a constant pressure chamber C and a variable pressure chamber D are defined across the rear diaphragm  13  within the rear chamber  5 . 
     A valve mechanism  15  is disposed within the valve body  6  and operates to switch a communication between the constant chambers A, C and the variable pressure chambers B, D. The valve mechanism  15  comprises an annular, first valve seat  16  formed around the valve body  6 , an annular, second valve seat  18  formed on the rear side of a valve plunger  17  which is slidably disposed within the valve body  6  at a location radially inward of the first valve seat  16 , and a valve element  20  adapted to be seated upon either valve seat  16  or  18  under the influence of a poppet return spring  19 . 
     A space radially outward of an annular seat area on the valve element  20  which moves into engagement with or disengagement from the first valve seat  16  communicates with the constant pressure chamber A through a first constant pressure passage  21  formed in the valve body  6 , and the interior of the constant pressure chamber A communicates with the constant pressure chamber C through a second constant pressure passage  22 . The constant pressure chamber A communicates with a source of negative pressure through a tubing (not shown) for introducing a negative pressure which is connected to the shell  2 , whereby the negative pressure is normally introduced into the constant pressure chambers A and C. 
     A space located radially inward of the annular seat area on the valve element  20  which moves into engagement with or disengagement from the first valve seat  16 , but radially outward of a seat area on the valve element  20  which moves into engagement with or disengagement from the second valve seat  18  communicates with the variable pressure chamber D through a radially extending, first variable pressure passage  24  formed in the valve body  6 , and the variable pressure chamber D communicates with the variable pressure chamber B through a second variable pressure passage  25  formed in the valve body  6 . 
     Finally, a space located radially inward of the annular seat area on the valve element  20  which moves into engagement with or disengagement from the second valve seat  18  communicates with the atmosphere through a pressure passage  26  formed in the valve body  6  and a filter  27  disposed therein. 
     The right end of the valve plunger  17  is pivotally connected with a distal end of an input shaft  30 , and a valve return spring  31  having a greater resilience than the poppet return spring  19  is disposed between the input shaft  30  and the valve body  6 , thus normally maintaining the valve element  20  seated upon the second valve seat  18  on the valve plunger while maintaining the valve element  20  away from the first valve seat  16  on the valve body  6 . The other end of the input shaft  30  is coupled to a brake pedal, not shown. 
     A key member  32  prevents a disengagement of the valve plunger  17  from the valve body  6 . The key member  32  is movable back and forth in the axial direction of the valve body  6 , and when the brake booster is inoperative, it abuts against the internal surface of the shell  2  to maintain the valve plunger  17  at an advanced position relative to the valve body  6 , thus reducing a lost motion of the input shaft  30  at the commencement of operation of the brake booster. 
     Reaction transmitting means  33 , a plate plunger  34  and a reaction disc  35 , all of which will be described in detail later, are sequentially disposed forwardly of the valve plunger  17 . The plate plunger  34  is slidably fitted in a holder  36  which is fixedly mounted on the front end face of the valve body  6  in a hermetically sealed manner, and the reaction disc  35  is received in a recess formed in one end of an output shaft  37 . The recess formed in one end of the output shaft  37  is slidably fitted around the outer peripheral surface of the holder  36 , and the front end of the output shaft  37  projects externally of the shell  2  for connection with a piston of a master cylinder, not shown, while maintaining a hermetic seal by means of a seal member  38 . 
     Accordingly, a brake reaction which is transmitted from the piston of the master cylinder is transmitted to the reaction disc  35  through the output shaft  37 , and is then accepted by the valve body  6  through the holder  36 , and is also transmitted to a brake pedal, not shown, through the plate plunger  34 , the reaction transmitting means  33 , the valve plunger  17  and the input shaft  30 . 
     The valve body  6  and the power pistons  10 ,  11  are normally maintained in their inoperative positions shown by a return spring  39  disposed between the shell  2  and the valve body  6 . 
     The reaction transmitting means  33  comprises a parallel combination of a first reaction transmitting path including a coiled spring  40 , and a second reaction transmitting path including a viscoelastic member  41  such as formed of rubber. 
     As shown to an enlarged scale in FIG. 2, the first reaction transmitting path including the coiled spring  40  comprises four components, namely, a rearwardly located, first retainer  42 , a forwardly located, second retainer  43 , the coiled spring  40  disposed between the both retainers  42  and  43 , and a stop member  44 . 
     The first retainer  42  which is rearwardly disposed includes an inner tubular portion  42   a  which is fitted around one end of a projection  17 A formed on the distal end of the valve plunger  17 , a radial portion  42   b extending radially outward from the rear end of the inner tubular portion  42   a  and abutting against a stepped end face of the valve plunger  17 , and an outer tubular portion  42   c  extending forwardly from the outer periphery of the radial portion  42   b.    
     On the other hand, the second retainer  43  which is located forwardly includes an outer tubular portion  43   a  which surrounds the first retainer  42 , a radial portion  43   b  extending radially inward from the front end of the outer tubular portion  43   a  and abutting against the rear end face of the plate plunger  34 , and an inner tubular portion  43   c  extending axially rearward from the inner periphery of the radial portion  43   b  and slidably fitted around the front end of the projection  17 A of the valve plunger  17 . 
     The coiled spring  40  is disposed between the radial portion  42   b  of the first retainer  42  and the radial portion  43   b  of the second retainer  43 , and the first retainer  42 , the second retainer  43 , the coiled spring  40  and the stop member  44  are integrally assembled together by causing the ring-shaped stop member  44  which is secured to the outer tubular portion  43   a  of the second retainer  43  to abut against the rear side of the first retainer  42  while maintaining the coiled spring  40  compressed to a given degree. 
     Accordingly, in the first reaction transmitting path, a brake reaction from the plate plunger  34  is transmitted to the valve plunger  17  through the second retainer  43 , the coiled spring  40  and the first retainer  42 . 
     On the other hand, the viscoelastic member  41  in the second reaction transmitting path is disposed inside the inner tubular portion  43   c  of the second retainer  43  and is disposed between the plate plunger  34  and the front end face of the projection  17 A of the valve plunger  17 . 
     Accordingly, in the second reaction transmitting path, a brake reaction from the plate plunger  34  is transmitted to the valve plunger  17  through the viscoelastic member  41 . 
     In the described arrangement, when the brake pedal is depressed to drive the input shaft  30  and the valve plunger  17  forward, the reaction transmitting means  33  is integrally driven forward while maintaining the condition shown in FIG. 2, and hence the plate plunger  34  is also driven forward until the force of depression exceeds the preset load to which the coiled spring  40  is charged. 
     As the valve plunger  17  is driven forward, a flow path in the valve mechanism  15  is switched to introduce the atmosphere into the variable pressure chamber B, whereby a pressure differential between the constant pressure chamber A and the variable pressure chamber B is effective to drive the power pistons  10 ,  11  and the valve body  6  forward to thereby drive the output shaft  37  forward, causing a braking liquid pressure to be generated in the master cylinder, generally in the similar manner as in a conventional brake booster. 
     A brake reaction which results from the braking liquid pressure is transmitted through the output shaft  37  to the reaction disc  35 , and thence through the plate plunger  34 , the reaction transmitting means  33 , the valve plunger  17  and the input shaft  30  to the brake pedal. 
     At this time, it will be seen that the spacing between the first retainer  42  and the second retainer  43  remains constant until the preset load to which the coiled spring  40  is charged is exceeded in the reaction transmitting means  33 , and accordingly, the spacing between the valve plunger  17  and the plate plunger  34  is also constant, whereby the viscoelastic member  41  cannot be compressed between the valve plunger  17  and the plate plunger  34 . 
     Thus, the force applied from the viscoelastic member  41  to the valve plunger  17 , or the brake reaction remains constant, and accordingly, the brake reaction is transmitted to the brake pedal only through the first reaction transmission path of the reaction transmitting means  33 . 
     At this time, the valve plunger  17  acts through the plate plunger  34  to compress the reaction disc  35 , so that the relationship between the force of depression upon release of the brake pedal and the brake booster  1  exhibits the hysteresis due to the presence of the reaction disc  35  which represents a viscoelastic member. 
     Subsequently, when the force with which the brake pedal is depressed increases to increase the output from the brake booster  1 , the brake reaction increases in a corresponding manner, and when the reaction exceeds the preset load to which the coiled spring  40  is charged, the first retainer  42  and the valve plunger  17  will be advanced relative to the second retainer  43  and the plate plunger  34  to reduce the spacing between the plate plunger  34  and the valve plunger  17 , as shown in FIG.  3 . Thus, when the brake reaction exceeds the preset load to which the coiled spring  40  is charged, both the coiled spring  40  and the viscoelastic member  41  are compressed. 
     When the coiled spring  40  and the viscoelastic member  41  are compressed, the first retainer  42  and the valve plunger  17  are advanced relatively by a corresponding amount, whereby the output from the brake booster  1  rises with a servo ratio which is greater than the servo ratio which prevailed previously. 
     At this time, the brake reaction is transmitted from the second retainer  43  through the compressed coiled spring  40  and the first retainer  42  to the valve plunger  17  in the first reaction transmission path of the reaction transmitting means  33 . 
     In the second reaction transmitting path, as the brake reaction increases, the viscoelastic member  41  becomes compressed, whereby the force applied from the viscoelastic member  41  to the valve plunger  17  increases, with consequence that the brake reaction is transmitted to the valve plunger  17  through the second reaction transmitting path as well as through the first reaction transmitting path. 
     In the conventional brake booster which is provided with conventional reaction transmitting means in which the viscoelastic member  41  is devoid, as the coiled spring  40  is compressed, the reaction disc  35  will be compressed and deformed. However, the hysteresis effect presented by the reaction disc  35  is accommodated by the coiled spring  40 , with result that the valve mechanism  15  switches a flow path in accordance with the extension and shrinkage of the coiled spring  40 . Since the coiled spring  40  exhibits little hysteresis effect, a brake feeling is degraded. 
     However, with the embodiment described above, as long as the coiled spring  40  is being compressed, the viscoelastic member  41  which exhibits the hysteresis is compressed at the same time, whereby the switching of the flow path by the valve mechanism retains the hysteresis effect, thus improving the brake feeling. 
     As the output from the brake booster  1  continues to increase, with a concomitant increase in the brake reaction, the viscoelastic member  41  presents a greater deformation resistance, whereby the amount of deformation of the coiled spring  40  and the viscoelastic member  41  is reduced, thus reducing the servo ratio. 
     If the deformation resistance presented by the viscoelastic member  41  increases very high so that the coiled spring  40  and the viscoelastic m ember  41  are no longer substantially deformed by compression, the reaction transmitting means  33  will become equivalent to a rigid body, whereby the servo ratio will be reduced to a value which prevailed before the preset load to which the coiled spring  40  is charged is exceeded. 
     When a choice is made so that the rear end of the second retainer  43  abuts against the valve body  6  while the coiled spring  40  continues to be compressed, the brake reaction which has been transmitted from the plate plunger  34  to the valve plunger  17  through the reaction transmitting means  33  will be accepted in its entirety by the valve body  6 , and hence the brake reaction which has been transmitted to the valve plunger  17  no longer increases. 
     As a consequence, if the brake pedal is further depressed, the brake reaction cannot exceed a given value, and hence, the valve plunger  17  cannot be pushed back against the valve body  6 , whereby the atmosphere continues to be introduced into the variable pressure chamber B to allow the output to be increased without increasing the force of depression, the brake booster  1  eventually reaching a full load condition. 
     In the described embodiment, the valve plunger  17  is formed with the projection  17 A which projects forwardly, and the viscoelastic member  41  is disposed between the projection and the plate plunger  34 . However, conversely, the projection  17 A may be omitted, while the plate plunger  34  may be formed with a projection which projects rearwardly, and the viscoelastic member  41  may be disposed inside the inner tubular portion  42   a  of the first retainer  42 . 
     FIG. 4 shows a second embodiment of the invention. In this embodiment, a valve plunger  117  has a projection  117 A in which an annular groove is formed to receive an O-ring  146  as a resilient member, which projects radially outward of the projection  117 A. A first retainer  142 , which forms reaction transmitting means  133 , has an inner tubular portion  142   a  which is disposed around the outer periphery of the O-ring  146  so as to compress it. Accordingly, the inner tubular portion  142   a  of the first retainer  142  is held in position by the O-ring  146 , whereby when the brake booster is inoperative, the entire reaction transmitting means  133  is held in place. 
     In the present embodiment, the outer diameter of the projection  117 A is slightly tapered in the rearward direction, and at the same time the inner tubular portion  142   a  of the first retainer  142  is similarly shaped, thus making the first retainer  142  less susceptible to withdrawal in the forward direction. 
     Accordingly, in the first reaction transmitting path, a brake reaction from a plate plunger  134  is transmitted through a second retainer  143 , a coiled spring  140  and the first retainer  142  to the valve plunger  117 . 
     On the other hand, a viscoelastic member  141  in the second reaction transmitting path is adhesively bonded to the front end face of the projection  117 A of the valve plunger  117 , and thus is disposed between the projection  117 A and the plate plunger  134 . Accordingly, in the second reaction transmitting path, a brake reaction from the plate plunger  134  is transmitted to the valve plunger  117  through the viscoelastic member. 
     In other respects, the arrangement is constructed in the similar manner as in the first embodiment, and accordingly, major parts which are similar to those shown in the first embodiment are designated by like reference numerals as used in the first embodiment, to which “100” is added. 
     In the present embodiment, not only the similar functioning and effect are achieved as in the first embodiment, but at the same time, the reaction transmitting means  133  is secured to the valve plunger  117  by means of the O-ring  146 , whereby it is possible to prevent a movement of the reaction transmitting means  133  back and forth in the axial direction to produce sounds of percussion when the brake booster is inoperative. 
     As shown in FIGS. 4 and 5, in the second embodiment, the outer periphery  144   a  of a ring-shaped stop member  144  is formed with a plurality of radially outwardly extending ears  144   b  at an equal interval circumferentially while the rear end of the outer tubular portion  143   a  of the second retainer  143  is formed with a plurality of slits  143   d  which are engaged by the respective ears  144   b , thereby providing the rear end of the outer tubular portion  143   a  which is located between the slits  143   d  to serve as a caulked portion  143   e.    
     Under the condition that the coiled spring  140  is disposed between the radial portion  142   b  of the first retainer  142  and the radial portion  143   b  of the second retainer  143 , and each ear  144   b  on the stop member  144  is held in abutment against the bottom surface of the respective slit  143   d  against the resilience of the coiled spring  140 , the caulked portion  143   e  is caulked by bending it radially inward, thus assembling the stop member  144 , the first retainer  142 , the second retainer  143  and the coiled spring  140  in an integral manner. 
     The inner peripheral surface of the stop member  144  is slidably fitted around the outer peripheral surface of the valve plunger  117  at a location rearward of the first retainer  142 , thus spacing the outer tubular portion  143   a  of the second retainer  143  from the outer tubular portion  142   c  of the first retainer  142 . 
     Accordingly, when the coiled spring  140  is compressed, the stop member  144  which is integral with the second retainer  143  slide s with respect to the valve plunger  117 , but because the stop member  144  is ringshaped, it is easy to form its sliding surface to a high precision , thus permitting the suppression of occurrence of extraneous sound during the sliding movement. 
     When caulking the s top member  144  to the second retainer  143 , the outer tubular portion  143   a  of the second retainer  143  is formed with the slit  143   d  to permit an abutment of the ear  144   b  of the stop member  144  against the end face thereof, thus allowing the outer diameter of the outer tubular portion  143   a  to be increased. 
     Specifically, when forming the slit  143   d  and the ear  144   b , if the outer tubular portion  143   a  of the second retainer  143  were formed with a portion of a greater diameter which is extended radially outward so that the outer periphery of the stop member  144  may be disposed in abutment against a stepped end face of the greater diameter portion, whereupon the rear part of the greater diameter portion is caulked, there results a portion of a greater diameter due to the outer tubular portion  143   a.    
     If it is desired to maintain the inner diameter of a region of the valve body  106  where the reaction transmitting means  133  is received constant, the use of the present embodiment which does not include a greater diameter portion permits the outer diameter of the outer tubular portion  143   a  to be increased, and thus the outer diameter of the coiled spring  140  which is received inside the outer tubular portion  143   a  can increased, thus enhancing the freedom of design of the coiled spring  140 . 
     FIG. 6 shows a third embodiment of the invention. In this embodiment, reaction transmitting means  233  comprises a retainer  242  which is mounted on the distal end of a valve plunger  217  in a displaceable manner, stop means  248  which is effective to prevent the retainer  242  from being withdrawn forwardly from the distal end of the valve plunger  217 , and a coiled spring  140  disposed between the retainer  242  and the valve plunger  217 . 
     The retainer  242  comprises an inner tubular portion  242   a , a radial portion  242   b  extending radially outward from the front end of the inner tubular portion  242   a  for abutment against the rear end face of a plate plunger  234 , and a fold-back  242   c  extending from the outer periphery of the radial portion  242   b  rearwardly. 
     The valve plunger  217  comprises a spring abutment  217   C  extending radially outward from a body  217 B and slidably fitted in a valve body  206 , and a projection  217 A extending forwardly from the distal end of the body  217 B. 
     The stop means  248  comprises a stop  217 D of a greater diameter which is formed at the distal end of the projection  217 A of the valve plunger  217 , and a detent  242   d  formed on the inner tubular portion  242   a  of the retainer  242  and projecting radially inward of the inner tubular portion  242   a.    
     The coiled spring  240  is disposed between the radial portion  242   b  of the retainer  242  and the spring abutment  217 C of the valve plunger  217  and is charged to a given load, and then the detent  242   d  formed on the inner tubular portion  242   a  is disposed in abutment against the rear end face  217 E of the stop  217 D, thus integrally assembling the retainer  242  and the coiled spring  240  together with the valve plunger  217 . 
     Accordingly, a brake reaction from the plate plunger  234  is transmitted through the retainer  242  and the coiled spring  240  to the valve plunger  217  in the first reaction transmitting path. 
     On the other hand, a viscoelastic member  241  in the second reaction transmitting path is disposed inside the inner tubular portion  243   c  of a second retainer  243 , and is disposed between the plate plunger  234  and the projection  217 A of the valve plunger  217 . Accordingly, a brake reaction from the plate plunger  234  is transmitted through the viscoelastic member  241  to the valve plunger  217  in the second reaction transmitting path. 
     In other respects, the arrangement is similar to the first embodiment, and accordingly, major parts which are similar to those shown in the first embodiment are designated by like reference numerals as used in the first embodiment, to which “200” is added. 
     According to the present embodiment, not only the similar functioning and effect can be achieved as in the first embodiment, but the reaction transmitting means  233  is assembled integrally into the distal end of the valve plunger  217 , thus permitting the suppression of a movement of the reaction transmitting means  233  back and forth in the axial direction to produce sounds of percussion when the brake booster is inoperative. 
     In addition, the valve plunger  217  is formed with the spring abutment  217   c , against which the rear end of the coiled spring  240  abuts directly, thus allowing the manufacturing cost to be reduced by an amount associated with the omission of the rear retainer in comparison to an arrangement in which the rear end of the coiled spring  240  abuts against the rear retainer. 
     FIG. 7 shows a fourth embodiment of the invention. In the third embodiment, the stop means  248  comprises the stop  217 D of the valve plunger  217  and the detent  242   d  on the retainer  242 . By contrast, in the present embodiment, stop means  341  comprises a stepped stop member  345  which is disposed as a press fit into a valve plunger  317 . Thus, in the present embodiment, the stop  217 D shown in the third embodiment is omitted, and a retainer  342  is slidably fitted around the outer periphery of a projection  317 A of the valve plunger  317 . 
     The retainer  342  comprises a tubular portion  342   a  slidably fitted into a bore  306   a  of a small diameter formed in a valve body  306 , a radial portion  342   b  extending radially outward from the rear end of the tubular portion  342   a  in an annular configuration, and a stepped through-opening  342   c  formed in the tubular portion  342   a  and having a greater diameter toward the front side. The projection  317 A of the valve plunger  317  is slidably fitted into a rearwardly located opening of a reduced diameter of the stepped through-opening  342   c.    
     An opening  317 E is formed in the distal end of the projection  317 A of the valve plunger  317 , and the stepped stop member  345  has a portion  345   a  of a small diameter which is secured by being a press fit into the opening  317 E. The stepped stop member  345  has a portion  345   b  of a greater diameter which may be utilized to compress a spring  340 , disposed between the radial portion  342   b  of the retainer  342  and the annular projection  317 C of the valve plunger  317 , to a degree while integrally assembling the retainer  342  and the spring  340  with the valve plunger  317 . 
     In other respects, the arrangement is similar to the third embodiment, and accordingly, major parts which are similar to those shown in the third embodiment are designated by like reference numerals as used in the third embodiment, to which “100” is added. 
     It is to be noted that the portion  345   a  of a small diameter of the stepped stop member  345  may be threadably engaged with the opening  317 E formed in the distal end of the projection  317 A, thus securing the stepped stop member  345  to the distal end of the projection  317 A. 
     FIG. 8 shows a fifth embodiment of the invention. In this embodiment, stop means  441  includes an annular stop member  445  which is secured to the distal end of a projection  417 A of a valve plunger  417 . Specifically, a retainer  442  is slidably fitted around the outer periphery of the projection  417 A of the valve plunger  417 , and the annular stop member  445  is fitted into an annular groove  417 F formed in the distal end of the projection  417 A, thus securing it in the axial direction. It should be understood that under this condition, a coiled spring  440  is compressed to a degree between a spring abutment  417 C of the valve plunger  417  and the radial portion  442   b  of the retainer  442 . 
     In other respects, the arrangement is similar to the fourth embodiment, and accordingly, major parts which are similar to those shown in the fourth embodiment are designated by like reference numerals as used in the fourth embodiment, to which “100” is added. 
     It should be understood that the annular stop member  445  may be caulked to the projection  417 A. When caulking the annular stop member  445 , the distal end of the projection  417 A may be previously formed with a tab of a small diameter, and the annular stop member  445  is fitted around the tab, and then the distal end of the tab may be squeezed to cause it to extend radially outward. 
     FIG. 9 shows a sixth embodiment of the invention which facilitates a caulking of an annular stop member  545  to a valve plunger  517 . 
     Specifically, a projection projects forwardly from the front end of a projection  517 A of the valve plunger  517  and is axially formed with a criss-cross slit  517 G. While fitting the annular stop member  545  around the projection thus formed, the distal end of this projection may be easily crushed and deformed utilizing the slit  517 G, thus securing the annular stop member  545  to the distal end of the projection  517 A of the valve plunger  517 . 
     In this embodiment, the valve plunger  517  has a spring abutment  517 C of a reduced diameter, and a plate member  546  is disposed on the spring abutment  517 C, with a spring  540  being compressed to a degree between the plate member  546  and the retainer  542 . 
     In addition, in this embodiment, a holder  536  is formed with an opening, in which a plate plunger  534  and the distal end of a tubular portion  542   a  of a retainer  540  are slidably fitted. 
     In other respects, the arrangement is similar to the fifth embodiment mentioned above, and accordingly, major parts which are similar to those shown in the fifth embodiment are designated by like reference numerals as used in the fifth embodiment, to which “100” is added. 
     FIG. 10 shows a seventh embodiment of the invention in which reaction transmitting means  633  is assembled initially and is then integrally assembled with a valve plunger  617 . 
     Specifically, the reaction transmitting means  633  comprises a rod member  647  which is equivalent to the projection in above embodiments, a retainer  642  slidably mounted on the front end of the rod member  647 , a plate member  646  disposed on the rear side of the rod member  647  and a coiled spring  640  disposed between the retainer  642  and the plate member  646 . 
     The rod member  647  includes a shank  647   a , a stop  647   b  of a greater diameter on the front side of the shank  647   a , and an annular recess  647   c  on the rear side of the shank  647   a.    
     The retainer  642  comprises an inner tubular portion  642   a  slidably fitted around the stop  647   b  of the rod member  647 , a radial portion  642   b  extending radially outward from the front end of the inner tubular portion  642   a  and abutting against the rear end face of a plate plunger  634 , a fold-back  642   c  extending rearwardly from the outer periphery of the radial portion  642   b , and a detent  642   d  formed on the rear portion of the inner tubular portion  642   a  to define stop means  648 . The detent  642   d  abuts against the rear end face of the stop  647   b  to prevent the retainer  642  from being withdrawn forwardly from the rod member  647 . 
     The plate member  646  comprises a securing portion  646   a  disposed toward the inner periphery thereof and tapering in the rearward direction, a radial portion  646   b  extending radially outward from the front end of the securing portion  646   a , and an outer tubular portion  646   c  extending forwardly from the outer periphery of the radial portion  646   b.    
     A coiled spring  640  is disposed between the retainer  642  and the plate member  646  and is charged to a given load, whereupon the retainer  642 , the coiled spring  640  and the plate member  646  are integrally assembled with the rod member  647 . The reaction transmitting means  633  assembled in this manner is secured to the valve plunger  617  by disposing the rear end of the shank  647   a  of the rod member  647  as a press fit into a fitting opening  617 H formed in the front end face of the valve plunger  617 . 
     In other respects, the arrangement is similar to the third embodiment mentioned above, and accordingly, major parts which are similar to those shown in the third embodiment are designated to like reference numerals as used in the third embodiment, to which “400” is added. 
     FIG. 11 shows an eighth embodiment of the invention. In the seventh embodiment, the securing portion  646   a  of the plate member  646  is tapered, but in the present embodiment, the securing portion is dispensed with, and the radial portion  746   b  of the plate member  746  is directly caulked to a shank  747   a  of a rod member  747  as shown at  747   e.    
     In other respects, an arrangement is similar to the seventh embodiment, and accordingly, major parts which are similar to those in the seventh embodiment are designated by like reference numerals as used in the seventh embodiment, to which “100” is added. 
     FIG. 12 shows a ninth embodiment of the invention. In the seventh embodiment, the rear end of the shank  647   a  of the rod member  647  is directly disposed as a press fit into the valve plunger  617 . However, in the present embodiment, a plate member  846  has a radial portion  846   b , the inner periphery of which is provided with a tubular portion  846   d  which is adapted to receive the rear end of a shank  847   a  of a rod member  847 . The tubular portion  846   d  is partly caulked while the rear end of the shank  847   a  is received within the tubular portion  846   d , thus integrally securing the plate member  846  to the rod member  847 . 
     The tubular portion  846   d  together with the rear end of the shank  847   a  of the rod member  847  is disposed as a press fit into a fitting opening  817 H formed in the valve plunger  817 , thus connecting reaction transmitting means  833  to the valve plunger  817 . 
     In other respects, the arrangement is similar to the seventh embodiment, and accordingly, major parts which are similar to those shown in the seventh embodiment are designated by like reference numerals as used in the seventh embodiment, to which “200” is added. 
     While the invention has been illustrated and described above in connection with several embodiments thereof, it should be understood that a number of changes, substitutions and modifications therein are possible from the above disclosure, and therefore it is intended that the invention be not limited to the specific disclosure given herein, but that the scope of the invention be solely defined by the appended claims.