Abstract:
In accordance with the invention, a booster is not provided with a mechanism which transmits a braking reaction, and accordingly, a reaction cannot be transmitted to a brake pedal. On the other hand, a pseudo-reaction imparting means is provided to impart a pseudo-reaction which depends on an amount of depression of the brake pedal. The pseudo-reaction imparting means imparts a pseudo-reaction having a reduced rate of increase to the brake pedal when a travel of the brake pedal is small, and imparts a pseudo-reaction having an increased rate of increase to the brake pedal when a travel of the break pedal is higher. This arrangement avoids the drawback of the prior art that an abnormally high braking reaction is transmitted to a driver as a result of an operational lag of a conventional booster during a quick braking operation. The invention prevents such braking reaction from being transmitted to a driver, but enables a reaction of a required magnitude to be transmitted to the driver by means of pseudo-reaction imparting means. In this manner, a required high braking effort can be obtained with a reduced force of depression during a quick braking operation. The pseudo-reaction imparting means which exhibit such characteristic is capable of realizing an operation feeling, as experienced in a normal operation in the prior art, independently from a quick and a slow braking operation.

Description:
This is a division of Ser. No. 08/859 739, filed May 21, 1997 now U.S. Pat. No. 6,003,426. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a booster as may be used in a brake of an automobile, and more particularly, to a reaction mechanism for booster which imparts reaction force to an operating rod which actuates the booster. 
     DESCRIPTION OF THE PRIOR ART 
     A brake booster is known in the art which comprises a valve body slidably disposed within a shell, a power piston mounted on the valve body, a constant and a variable pressure chamber defined across the power piston, a valve mechanism disposed in the valve body, an input shaft connected to a brake pedal serving as an operating rod and adapted to drive a valve plunger, which forms part of the valve mechanism, back and forth to thereby switch a flow path in the valve mechanism, and an output shaft driven forward as the valve body is driven forward to cause a piston of a master cylinder to be driven forward, thereby developing a braking liquid pressure. 
     A conventional reaction mechanism is provided with a reaction disc formed of rubber and disposed between the output shaft and the valve plunger so that when the brake booster is actuated, both the valve body and the valve plunger are brought into contact with the reaction disc simultaneously, part of a braking force applied to the output shaft being transmitted to the valve body while the remainder thereof is transmitted to the valve plunger, the braking reaction force transmitted to the valve plunger being sensed by a driver through the input shaft and the brake pedal. 
     A serve ratio of the brake booster can be changed by changing the ratio of the reaction force transmitted to the valve body and the reaction force transmitted to the valve plunger, and more specifically, the ratio of pressure-responsive areas of the both members. A high serve ratio is generally established in the brake booster so that a braking liquid pressure of an increased magnitude can be developed with a depression of the brake pedal with a reduced force. However, it is found that during a quick braking operation, a serve ratio of a predetermined magnitude cannot be obtained as a result of an operational lag of the brake booster, and it is difficult for a less powerful driver such as an aged person or woman to perform a quick braking operation. 
     More specifically describing the reason, when the brake pedal is depressed, a flow path in the valve mechanism is switched through the input shaft, whereby a pressure fluid is introduced into the variable pressure chamber to drive the power piston and the valve body forward. As the valve body is driven forward, the output shaft is also driven forward through the reaction disc, the forward movement of the output shaft developing a braking pressure. A reaction therefrom is applied to the output shaft, and the brake reaction applied to the output shaft is distributed between the valve body and the valve plunger in a manner mentioned above. However, the valve plunger which is connected to the brake pedal through the input shaft will be driven forward before the power piston and the valve body are driven forward by the pressure fluid which is introduced into the variable pressure chamber during a quick braking operation, and as a consequence, a major proportion of the braking reaction which is applied to the output shaft will be transmitted to the valve plunger. This results in a braking reaction of an abnormally high magnitude being transmitted to the driver. Consequently, when a quick braking operation is to be performed, the brake pedal must be depressed by overcoming the abnormal braking reaction which is transmitted. As compared with a normal braking operation in which the brake pedal is gradually depressed to achieve a high braking force, a braking force of an increased magnitude which is required for a quick braking operation cannot be obtained unless the brake pedal is depressed with a force which is much greater than in a normal braking operation. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, the invention provides a reaction mechanism for booster which enables a high output to be obtained with a reduced force of depression during a quick braking operation without degrading a normal operation feeling of the booster. 
     Thus, the invention relates to a booster including a valve body slidably disposed within a shell, a power piston mounted on the valve body, a constant and a variable pressure chamber defined across the power piston, a valve mechanism disposed in a valve body, an input shaft connected to an operating rod for driving a valve plunger, which forms a valve mechanism, back and forth to switch a flow path in the valve mechanism, and an output shaft which is driven forward as the valve body is driven forward. In accordance with the invention, there is provided pseudo-reaction imparting means which prevents the reaction applied to the output shaft during the operation of the booster from being transmitted to the valve plunger and which imparts a pseudo-reaction which depends on a travel of the input shaft to the operating rod. The pseudo-reaction imparting means is arranged such that a pseudo-reaction having a lower rate of increase is imparted to the operating rod when a travel of the operating rod is low, while a pseudo-reaction having a higher rate of increase is imparted to the operating rod when a travel of the operating rod is higher. 
     With the described arrangement, upon actuation of the booster, the reaction applied to the output shaft is prevented from being transmitted to the valve plunger, and accordingly, during a quick braking operation, if the valve plunger which is connected to the operating rod through the input shaft is driven forward before the power piston and the valve body are driven forward by pressure fluid introduced in the variable pressure chamber, the transmission of a reaction applied to the output shaft to a driver through a path including the valve plunger, the input shaft and the operating rod is prevented. 
     On the other hand, the pseudo-reaction imparting means transmits a pseudo-reaction which depends on a travel of the operating rod to the driver through the operating rod. In this manner, the transmission of an abnormally high reaction during a quick braking operation is prevented, and instead, a high output can be obtained with a reduced operating force in the same manner as in a usual operation. 
     As mentioned above, the pseudo-reaction imparting means imparts a pseudo-reaction having a lower rate of increase to the operating rod when a travel of the operating rod is low, and imparts a pseudo-reaction having an increased rate of increase to the operating rod when a travel of the operating rod is high. Accordingly, a similar operation feeling as in a usual operation can be obtained independently from a quick and a slow braking operation. By way of example, when an operating rod comprises a brake pedal, during an initial phase of the braking operation, an increase in the pseudo-reaction will be low if the brake pedal is depressed through an increased stroke, but after a braking force of certain magnitude is developed, the magnitude of pseudo-reaction will increase relative to the amount of depression of the brake pedal if it is small. The pseudo-reaction imparting means which exhibits such characteristic is effective in realizing such a normal operation feeling. 
    
    
     Above and other objects, features and advantages of the invention will become apparent from the following description 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 of a second embodiment of the invention; 
     FIG. 3 is a cross section of a third embodiment of the invention; 
     FIG. 4 is a cross section of a fourth embodiment of the invention; 
     FIG. 5 is an enlarged view of part shown in FIG. 4; 
     FIG. 6 is a cross section taken along the line V—V shown in FIG. 5; 
     FIG. 7 graphically shows the characteristic diagram of a fourth embodiment; 
     FIG. 8 is a schematic view of a fifth embodiment of the invention; 
     FIG. 9 is a front view of a sixth embodiment of the invention; and 
     FIG. 10 is a side elevation of FIG.  9 . 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Referring to the drawings, several embodiments of a brake booster to which the invention is applied will be described. Referring to FIG. 1, a front shell  1  and a rear shell  2  constitute together an enclosed vessel, in which a substantially tubular valve body  3  is slidably disposed. A power piston  4  is mounted around the outer periphery of the valve body  3  and a diaphragm  5  is applied to the rear side thereof, thus partitioning the interior of the enclosed vessel into a forwardly located constant pressure chamber  6  and a rearwardly located variable pressure chamber  7 . 
     The valve body  3  contains a valve mechanism  9  which switches a fluid circuit. Specifically, the valve mechanism  9  comprises an annular first valve seat  10  formed around the inner periphery of the valve body  3 , an annular second valve seat  12  formed on the right end of a valve plunger  11  which is slidably fitted in the valve body  3 , and a valve element  14  which is urged from the right, as viewed in FIG. 1, to be seated upon either valve seat  10  or  12  under the influence of a poppet return spring  13 . 
     A space located radially outward of the first valve seat  10  communicates with the constant pressure chamber  6  through an axial constant pressure passage  15  formed in the valve body  3 , and the constant pressure chamber  6  communicates with an intake manifold of an engine through a tubing  16  mounted on the front shell  1  for introducing a negative pressure. In this manner, a negative pressure is normally introduced into the chamber  6 . A space located intermediate the first valve seat  10  and the second valve seat  12  communicates with the variable pressure chamber  7  through a radial variable pressure passage  17  formed in the valve body  3 . Finally, a space located radially inward of the second valve seat  12  communicates with the atmosphere through a pressure passage  18  formed in the valve body  3 . 
     The right end of the valve plunger  11  is pivotally connected with a distal end of the input shaft  21 , the other end of which is connected to a brake pedal (operating rod), not shown. The plunger  11  is prevented from being disengaged from the valve body  3  by means of a key member  22 . While not shown, the key member  22  is forked in a region from its center toward the distal end thereof, and the key member  22  is inserted into a receiving hole  23  which is diametrically formed in the valve body  3 , with the end from which the forked portion extends is engaged with a portion  11   a  of the valve plunger which has a reduced diameter. 
     It is to be noted that the key member  22  is displaceable axially of the valve body  3  within the hole  23 , and the key member  22  and the valve plunger  11  are displaceable axially of the valve body  3  within an extent of the axial length of the portion  11   a . When the brake booster is inoperative, the key member  22  is held in abutment against the inner surface of the rear shell  2  to maintain the key member  22  and the valve plunger  11  at an advanced position relative to the valve body  3  so that a lost motion of the input shaft  21  at the commencement of operation of the brake booster can be reduced. 
     Formed in the right end of an output shaft  14  and which is disposed to the left of the valve plunger  11  is a recess  24   a  in which an elastic member  25  formed of rubber and a spacer  26  formed of a metal disc are sequentially fitted, and an annular projection  3   a  formed to extend from the right end of the valve body  3  is also slidably fitted in the recess  24   a.    
     The left or the distal end of the output shaft  24  projects externally of the front shell  1  with a seal  27  maintaining a hermetic seal, and the distal end is connected to a piston of a master cylinder,  24   b , which is connected to the front shell  1 . It is to be noted that the valve body  3  and the power piston  4  are normally maintained in the inoperative position shown by a return spring  28 . 
     It is to be understood that in a conventional practice, the spacer  26  has been omitted, and the left end face of the valve plunger  11  is disposed in opposing relationship with the elastic member  25 , serving as a reaction disc, with a very slight clearance therebetween so that when the brake booster is actuated, the brake reaction transmitted from the output shaft  24  can be transmitted to both the valve body  3  and the valve plunger  11  through the elastic member  25 , and the braking reaction transmitted to the valve plunger  11  is transmitted to a driver input through the input shaft  21  and a brake pedal (operating rod), not shown. 
     By contrast, in the present embodiment, the spacer  26  is included, and an increased spacing is provided between the spacer  26  and the valve plunger  11  so that an abutment therebetween can be avoided even during a quick braking operation. In this manner, the entire braking reaction which is transmitted from the output shaft  24  is transmitted to the valve body  3  through the elastic member  25  and the spacer  26  while no braking reaction is transmitted to the valve plunger  11 , and hence to the brake pedal. In the present embodiment, the elastic member  25  is used as an impact buffering member. 
     The described arrangement alone cannot provide a brake feeling to a driver. Accordingly, a pseudo-reaction imparting means  31  is provided to impart a pseudo-reaction which depends on an amount of depression of the brake pedal to a driver. 
     The pseudo-reaction imparting means  31  utilizes a valve return spring  32  which urges the input shaft  21  rearwardly. In the conventional practice, a valve return spring is disposed between the valve body  3  and the input shaft  21 , but in the present embodiment, it is disposed between a tubular member  33  which is integral with the carrosserie (a part of a car body) and the input shaft  21 . 
     The tubular member  33  comprises an outer tubular portion  33   a,  an inner tubular portion  33   b  which is inserted into the valve body  3  from the right side thereof, a connector portion  33   c  which joins both tubular portions  33   a,    33   b  at their right ends, and a flange  33   d  which projects radially inward from the left end of the inner tubular portion  33   b.  The rear shell  2  includes, at its center, a tubular portion  2   a  which extends rearwardly and which is disposed as a press fit within the outer tubular portion  33   a  while the left end face of the outer tubular portion  33   a  is held in abutment against the outer surface of the rear shell  2  simultaneously, thus integrally connecting the tubular members  33  and the rear shell  2  together. The rear shell  2  is mounted on the carrosserie, not shown, by means of a mounting bolt  34  shown mounted thereon, and therefore it will be seen that the tubular member  33  is fixedly mounted on the carrosserie through the rear shell  2 . A retainer  35  is fixedly mounted on the input shaft  21 , and the valve return spring  32  is disposed between the flange  33   d  of the tubular member  33  and the retainer  35  to urge the input shaft  21  rearward. 
     With the described arrangement, when the brake pedal is depressed to drive the input shaft  21  and the valve plunger  11  forward, a flow path within the valve mechanism  9  is switched to introduce the atmosphere into the variable pressure chamber  7  in the similar manner as in a conventional brake booster. Accordingly, a pressure differential between the constant pressure chamber  6  and the variable pressure chamber  7  is effective to drive the power piston  4  and the valve body  3  forward, thus causing the output shaft  24  to be driven forward to develop a braking liquid pressure in a master cylinder. 
     A braking reaction which is produced by the braking liquid pressure is entirely transmitted to the valve body  3  through the output shaft  24 , the elastic member  25  and the spacer  26 , and no portion thereof is transmitted to the valve plunger  11 . 
     On the other hand, when the brake pedal is depressed to drive the input shaft  21  forward, it will be seen that the input shaft  21  moves forward while compressing the valve return spring  32  against the flange  33   d  of the tubular member  33  which is fixedly mounted on the carrosserie, and accordingly, there results a pseudo-reaction which is proportional to the compression of the valve return spring  32 . This pseudo-reaction is transmitted to a driver through the input shaft  21  and the brake pedal. Accordingly, by choosing an optimum condition for a preset load, preset length or spring constant of the valve return spring  32 , there can be obtained an excellent brake feeling comparable to the prior art for the driver. 
     It will be seen that the pseudo-reaction imparting means  32  imparts a pseudo-reaction to the brake pedal, which remains substantially the same during a quick braking operation as in a normal braking operation, thus avoiding the occurrence of an abnormally high braking reaction as experienced with a conventional arrangement during a quick braking operation. In this manner, a high braking output can be obtained with a relatively low force of depression during a quick braking operation, and a reliable quick braking operation can be assured for a powerless driver such as an aged person or woman. 
     It is to be noted that the elastic member  25  and the spacer  26  may be omitted. 
     Second Embodiment 
     FIG. 2 shows a second embodiment of the invention in which a pseudo-reaction imparting means  131  is contained within the brake booster. It should be noted that parts and members corresponding to those shown in FIG. 1 are designated by like numerals as used before to which  100  is added. 
     The pseudo-reaction imparting means  131  includes a reaction piston  138  disposed forwardly of a valve body  103 . The reaction piston  138  includes a portion  138   a  of a reduced diameter located at the right end, a pair of arms  138   b  extending radially outward from the left end of the portion  138   a , and a tubular portion  138   c  extending from the distal ends of arms  138   b . The portion  138   a  is slidably fitted into the valve body  103 , from the front side, while maintaining a hermetic seal therebetween. When the brake booster is inoperative, a valve plunger  111  is substantially in abutment against the portion  138   a.    
     Formed in the right end of an output shaft  124  is a recess  124   a , in which an annular projection  103   a  formed at the right end of the valve body  103  is fitted while the right end face of the output shaft is held in abutment against the valve body  103 , thus connecting the both members integrally or in a manner to prevent a relative displacement therebetween. 
     The right end of the output shaft  124  (FIG. 2) is formed with slits  124   b  which extend axially thereof, thus allowing the arms  138   b  of the reaction piston  138  to extend therethrough. In this manner, an axial displacement of the reaction piston  138  with respect to the output shaft  124  is permitted. A spring  139  is provided between the tubular portion  138   c  of the reaction piston  138  and a front shell  101  to produce a pseudo-reaction. 
     In other respects, the arrangement is similar to the first embodiment. In the present embodiment, when the booster is inoperative as shown, or when a key member  122  abuts against the inner surface of a rear shell  102  to maintain the key member  122  and a valve plunger  111  at an advanced position relative to the valve body  103 , the valve plunger  111  abuts the against the portion  138   a  of the reaction piston  138 . When the brake pedal is depressed under this condition to drive an input shaft  121  and the valve plunger  111  forward, the spring  139  will be compressed, whereby a pseudo-reaction having a magnitude which is proportional to the compression of the spring is transmitted to a driver through the reaction piston  138 , the valve plunger  111  and the input shaft  121 . 
     On the other hand, the braking reaction which is applied to the output shaft  124  is entirely transmitted to the valve body  103 , and no portion thereof is transmitted to the valve plunger  111 , and hence to the brake pedal. 
     As a consequence, an excellent brake feeling which is comparable to the prior art is assured for the driver, and a reliable quick braking operation is assured for a physically weaker driver such as an aged person or woman. 
     When the brake booster is released, or when the brake pedal is released, the retracting movement of the reaction piston  138  is limited by abutment against the valve body  103 , but the valve plunger  111  is allowed to retract relative to the valve body  103  until its retracting movement is limited by the key member  122 , whereby the valve plunger  111   b  will be spaced from the portion  138   a  having a reduced diameter of the reaction piston  138 . 
     When the pressure within the variable pressure chamber  107  is reduced as a result of switching the flow path of the valve mechanism  109 , a power piston  104  and the valve body  103  retract under the influence of a return spring  128 . The retracting movement will be limited initially when the key member  122  abuts against the inner surface of the rear shell  102 , but the valve body  103  continues to retract until it abuts against the key member  122  where it comes to a stop. 
     Under this condition, the key member  122  and the valve plunger  111  which is associated therewith will be maintained at an advanced position relative to the valve body  103 , and return to their inoperative conditions as the valve plunger  111  moves forward when the distal end thereof abuts against the portion  138   a  of the reaction piston  138 . 
     Third Embodiment 
     FIG. 3 shows a third embodiment of the invention where it is to be noted that corresponding parts or members as shown in FIG. 1 are designated by like numerals as used before to which  200  is added. 
     In the present embodiment, a pseudo-reaction imparting means  231  comprises a spring  243  disposed between a brake pedal  241  and a carrosserie  242 . Specifically, a clevis  244  is mounted on one end of an input shaft  221 , and the clevis  244  and the brake pedal  241  are connected together by a clevis pin  245 . One end of the spring  243  is engaged with the pin  245 . It is to be noted that the distal end of the brake pedal  241  is connected to a carrosserie  242  in a rockable manner by a connecting pin  246 . Rather than utilizing the clevis pin  245 , the spring  243  may be directly connected to the brake pedal  241  or to the input shaft  241 . It will be apparent that the use of such pseudo-imparting means  231  is effective to achieve the functioning as mentioned above. 
     It is to be noted that in the present embodiment, an opening in which the distal end of a valve plunger  211  is slidably fitted so as to prevent a braking reaction applied to an output shaft  224  from being transmitted to the valve plunger  211  is not a through-opening as shown in the described embodiments, but has its end located toward the output shaft  224  blocked. 
     In the described embodiments, pseudo-reaction imparting means  31 ,  131  and  231  utilize springs  32 ,  139  and  243  as elastic members, but the use of such springs is not essential, but an elastic member such as formed of rubber may also be used. 
     Fourth Embodiment 
     FIGS. 4 to  6  show a fourth embodiment of the invention in which a first spring  336  and a second spring  342  constitute together a pseudo-reaction imparting means  331  of the present embodiment in contra-distinction to the previous embodiments in which a single spring  32 ,  139  or  243  is used. It is to be understood that the essential parts of the brake booster according to the fourth embodiment shown in FIG. 4 are similar in construction to that of a brake booster shown in the first embodiment, and accordingly, corresponding parts or members are designated by like reference numerals as used in the first embodiment to which  300  is added. 
     Referring to FIG. 5, a brake pedal  321  has its one end connected to a carrosserie  333  in rockable manner by means of a connecting pin  332 . A first clevis  334  is mounted on one end of an input shaft  319 , and a clevis pin  335  extends through the first clevis  334  and through the brake pedal  321 , thus connecting the input shaft  319  and the brake pedal  321  together. 
     As shown in FIGS. 5 and 6, the first spring  336  having a reduced resilience is disposed between the clevis pin  335  and the carrosserie  333 , and the tension of the first spring  336  is effective to hold the brake pedal  321  in abutment against a first stop  337  (FIG. 5 ) mounted on the carrosserie  333  normally. Rather than connecting it to the clevis pin  335 , the first spring  336  may be directly connected to the brake pedal  321 , to the first clevis  334  or to the input shaft  319 . 
     The first clevis  334  is provided within a second clevis  341  so as to be reciprocable axially of the input shaft  319 , and the opposite ends of the clevis pin  335  are engaged with elongate slots  341   a  formed axially of the input shaft  319  in the second clevis  341 , whereby a relative movement between the first clevis  334  and the second clevis  341  is possible within an extent of the length of the elongate slots  341   a.  The second spring  342  having a resilience which is greater than the resilience of the first spring  336  is disposed between the second clevis  341  and the carrosserie  333  and extends in a parallel relationship with the first spring  336 . The tension of the spring  342  is effective to maintain the second clevis  341  normally in abutment against a second stop  343  provided on the carrosserie  333 . Under this condition, the clevis pin  335  lies toward the rear end of the elongate slots  341   a.    
     With the described arrangement, when the brake pedal  321  is depressed to drive the input shaft  319  and a valve plunger  311  forward, a flow path in a valve mechanism  309  is switched to introduce the atmosphere into a variable pressure chamber  307  in the similar manner as in a conventional booster, whereby a pressure differential between a constant pressure chamber  306  and the variable pressure chamber  307  drives a power piston  304  and a valve body  303  forward to thereby drive the output shaft  324  forward, causing a braking liquid pressure to be developed in a master cylinder. A braking reaction which results from the braking liquid pressure is entirely transmitted to the valve body  303  through the output shaft  324 , an elastic member  325  and a spacer  326 , but not transmitted to the valve plunger  311 . 
     On the other hand, when the brake pedal  321  is depressed, it initially causes the first spring  336 , and then the second spring  342  to be tensioned, whereby a pseudo-reaction which depends on such tension is produced on the brake pedal  321 . 
     Specifically, when the brake pedal  321  is depressed, it is initially subject to the resilience of the spring  336 . As the brake pedal  321  is depressed further, and the clevis pin  335  which is integral with the brake pedal  321  moves to the forward end of the elongate slots  341   a  formed in the second clevis  341 , the clevis pin  335  causes the second clevis  341  to be driven forward against the resilience of the second spring  342 , whereupon the resilience of both the first spring  336  and the second spring  342  which are disposed parallel to each other acts simultaneously upon the brake pedal  321 . 
     A resulting relationship between the stroke of the brake pedal  321  and a force of depression is graphically shown in FIG. 7, where a characteristic represented by a straight line A is produced by the first spring  336  and has a slope which is determined by a spring constant k 1  of the first spring  336 , and an intercept B with the abscissa is determined by the preset load upon the first spring  336 . It will be seen from this characteristic represented by the straight line A that because the resilience of the first spring  336  is chosen to be small, the stroke increases rapidly in response to a small increase in the force of depression of the brake pedal  321  during the initial phase of the braking operation. 
     A point C shown in FIG. 7 represents a beginning point when the resilience of both the first spring  336  and the second spring  342  acts upon the brake pedal  321  simultaneously. A characteristic represented by a straight line D is produced by a combination of the first spring  336  and the second spring  342 , and has a slope which is determined by a combined effect of a spring constant k 1  of the first spring  336  and a spring constant k 2  of the second spring constant  342 . The combined spring constant k is represented as: k=k 1 +k 2 . Because the resilience of the second spring  342  is chosen to be greater than that of the first spring  336 , subsequent to the point C, an increase in the stroke can be reduced in response to a relatively large increase in the force of depression. 
     It is to be understood that a more excellent brake feeling can be obtained by a combination of the straight lines A and D mentioned above rather than determining the relationship between the stroke and the force of the depression in accordance with a single straight line or a single spring. 
     It is to be understood that the pseudo-reaction imparting means  331  imparts a pseudo-reaction to the brake pedal  321 , which remains substantially unchanged between the quick and the normal braking operation, avoiding the occurrence of an abnormally high reaction during a quick braking operation as experienced with a conventional arrangement. Accordingly, a high braking output can be obtained with a relatively small force of depression during a quick braking operation, and thus a reliable quick operation is assured for a powerless driver such as an aged person or woman. 
     Fifth Embodiment 
     FIG. 8 shows a fifth embodiment of the invention in which a pair of springs are disposed in series rather than in a parallel relationship as shown in the fourth embodiment. As before, parts and members corresponding to those shown in FIG. 1 are designated by like numerals as used in FIG. 1 to which  400  is added. 
     In the present embodiment, a pseudo-reaction imparting means  431  comprises a pair of a first spring  436  and a second spring  442  which are disposed in series between a brake pedal  421  and a carrosserie  433  and which are connected to each other. A stop plate  451  is provided at the juncture between the both springs  436 ,  442 , and the stop plate is disposed for abutment against a stop  452  mounted on the carrosserie  433 . 
     In the inoperative condition, the stop plate  451  is spaced a given distance from the stop  452 , and when the brake pedal  421  is depressed under this condition, the both springs  436 ,  442  are tensioned to cause the stop plate  451  to abut against the stop  452 , whereupon only the first spring  436 , which is located nearer to the brake pedal  421  will be tensioned. A combined spring constant k when a pair of springs which are connected in series are tensioned is given as k=k 1 ·k 2 /(k 1 +k 2 ) where k 1  and k 2  represent the spring constant of the respective springs. It will be apparent from this equation that the combined spring constant k will be reduced than any one of the spring constants k 1  and k 2  of the respective springs, and such a combined spring constant k having a reduced magnitude can be used to achieve the characteristic as represented by the straight line A shown in FIG.  7 . 
     On the other hand, subsequent to the abutment of the stop plate  451  against the stop  452 , only the first spring  436  will be tensioned, thus producing the characteristic as represented by the straight line D shown in FIG. 7 in accordance with the spring constant k 1 . 
     In each of the described embodiments, each single spring  336 ,  342 ,  436 ,  442  may comprise a plurality of springs which are connected together in series or in parallel, thereby providing a required combined spring constant in a flexible manner. 
     Sixth Embodiment 
     FIGS. 9 and 10 show a sixth embodiment of the invention in which a pseudo-reaction imparting means  531  comprises a first spring which utilizes the valve return spring  20  shown in FIG. 1 and a second spring which comprises a torque spring  542 . As before, parts and members corresponding to those shown in FIG. 1 are designated by like reference numerals as used in FIG. 1 to which  500  is added. 
     The torque spring  542  is disposed in surrounding relationship with a connecting pin  532  which connects a brake pedal  521  to a carrosserie  533 . One end  542   a  of the torque spring abuts against a stop  555  mounted on the brake pedal  521  while the other end  542   b  is engaged with an arcuate groove  556   a  formed in a bracket  556  which forms part of the carrosserie  533 . The arcuate groove  556   a  is formed to be arcuate about the connecting pin  532 . 
     The brake pedal  521  is normally maintained such that the stop  555  is in abutment against a stop  557  which is mounted on the carrosserie  533 . Under this condition, the other end  542   b  of the torque spring  542  is spaced a given distance δ from the end of the arcuate groove  556   a.    
     When the brake pedal  521  is depressed under this condition, an input shaft is driven forward against the resilience of the valve return spring  20 , and subsequently when the other end  542   b  of the torque spring  542  abuts against the end of the arcuate groove  556   a , the resilience of the torque spring  542  becomes acting upon the brake pedal  521 . 
     Accordingly, a characteristic as represented by the straight line A shown in FIG. 7 is achieved by the valve return spring  20  while a characteristic as represented by the straight line D as shown in FIG. 7 can be achieved by a combination of the valve return spring  20  and the torque spring  542 . 
     In the described embodiments, the brake pedal  241 ,  321 ,  421  or  521  is utilized as an operating rod, but the operating rod may also comprise a manual brake lever which is provided for a physically handicapped person. It should be understood that the invention is also applicable to a clutch booster where a clutch pedal is used as an operating rod. 
     While the invention has been disclosed above in connection with several embodiments thereof, it should be understood that these embodiments are illustrative only, and that a number of changes, modification and substitutions therein are possible without departing from the spirit and the scope of the invention as defined by the appended claims.