Patent Publication Number: US-2015082787-A1

Title: Master cylinder

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     This patent application is a divisional of co-pending U.S. patent application Ser. No. 13/159,948, filed Jun. 14, 2011, which is a continuation of international application number PCT/EP2009/067205 filed on Dec. 15, 2009, which claims the benefit of German application No. 10 2008 063 241.4, filed Dec. 16, 2008, the entire teachings and disclosures of which are incorporated herein by reference thereto. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a master cylinder, in particular for a clutch, actuating or brake system of a vehicle. 
     BACKGROUND OF THE INVENTION 
     DE 10 2004 055 410 discloses a master cylinder of this type, wherein the piston of this master cylinder must pass through a considerable distance in order to be able to build up pressure in the cylinder chamber from the pressure equalization position. 
     The problem with such master cylinders is to make the idle stroke which occurs as small as possible. The idle strike is the stroke which is necessary in order to be able to build up pressure in the cylinder chamber proceeding from the pressure equalization position. 
     Embodiments of the invention address the forgoing problems. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein. 
     SUMMARY OF THE INVENTION 
     Embodiment of the invention include a master cylinder, in particular for a clutch, actuating or brake system of a vehicle, comprising a cylinder housing with a piston bore, a piston which is arranged in the piston bore and has an end side facing a cylinder chamber and at least one afterflow opening which leads from a piston skirt surface through a piston wall to the cylinder chamber at a distance from the end side and adjoins an afterflow space in a pressure equalization position of the piston so that in the pressure equalization position hydraulic medium can run on into the cylinder chamber in order to equalize the pressure, and an inner sealing element which is arranged between the housing and the piston in the area of the piston bore and limits the cylinder chamber. 
     This object is accomplished in accordance with the invention, in a master cylinder of the type described at the outset, in that the afterflow space is arranged between the sealing element and the piston skirt surface on a side of a sealing lip of the sealing element facing away from the cylinder chamber and that in the pressure equalization position the afterflow opening is arranged so as to be located opposite an inner surface of the sealing element which limits the afterflow space. 
     The advantage of the solution according to the invention is to be seen in the fact that it is possible, as a result of moving the afterflow space into an area between the sealing element and the piston skirt surface, to position the at least one afterflow opening in the pressure equalization position such that it passes through below the sealing lip, which abuts sealingly on the piston skirt surface, with even a slight stroke movement of the piston and, therefore, pressure can be built up in the cylinder chamber. 
     As a result, the idle stroke is minimized in accordance with the object described at the outset. 
     In this respect, it is particularly favorable when the sealing element has inner surface areas extending at a distance from the piston skirt surface for the formation of the afterflow space. 
     Such inner surface areas extending at a distance from the piston skirt surface create the possibility of providing a sufficiently large volume for the afterflow space. 
     One advantageous solution provides for the inner surface areas to form a cavity which extends around the piston skirt surface and can, for example, be worked into the sealing element as an annular groove. 
     Such a cavity creates the possibility of communicating with a plurality of afterflow openings in the piston so that a sufficiently large afterflow cross section can be made available. 
     Another advantageous solution provides for the inner surface areas to form overflow channels which lead to a side of the sealing element which faces away from the cylinder chamber. Overflow channels of this type offer the possibility not only of providing as large a volume as possible for the afterflow space but also of providing a sufficiently large flow cross section for the hydraulic medium to run on. 
     The overflow channels could extend, for example, spirally or helically in the sealing element. 
     One particularly favorable solution provides, however, for the overflow channels to extend approximately parallel to a direction of movement of the piston. 
     In order, on the one hand, to ensure the supply of hydraulic medium to the afterflow space and in order, on the other hand, to bring about a reliable discharge of air bubbles from the afterflow space, it is provided for the afterflow space to lie adjacent to at least one supply recess, via which hydraulic medium can be supplied to the afterflow space and via which air bubbles can also exit from the afterflow space. 
     It is provided, in particular, for the supply recess to vary in a radial direction towards the afterflow space. 
     A feeding of hydraulic medium to the afterflow space can be realized favorably, in particular, when the supply recess is in communication with a supply channel. 
     It is particularly favorable when the afterflow space lies adjacent to a supply gap which is located between the piston skirt surface and the piston bore and via which the hydraulic medium can flow on into the afterflow space. 
     The supply gap can be of a cylindrical design. It is particularly favorable when the supply gap widens in a radial direction towards the afterflow space. 
     One particularly simple solution provides for the supply gap to widen conically. 
     The supply gap can widen in the direction of the afterflow space at an angle of between 0.5° and 45°. It is particularly favorable when the angle is in the range of 1° to 8°. 
     The supply gap preferably has a radial extension in the range of 0.01 mm and 0.5 mm. 
     Furthermore, the supply gap preferably has a gap length extending parallel to a direction of movement of the piston of 0.1 mm to 10 mm. 
     Alternatively to the provision of a supply gap, it is possible to provide at least one supply section in a wall area between the afterflow space and the supply channel or a hydraulic chamber. 
     Preferably, a plurality of supply sections which are arranged at a distance from one another will be provided in the wall area. 
     The at least one supply section is, in particular, dimensioned such that its radial extent becomes increasingly smaller with increasing extension in the direction of the afterflow space and so as small a gap as possible in relation to the piston skirt surface exists on the side of the afterflow space. 
     In this respect, the overflow channels form, in particular, a connection between the cavity and the supply recess. 
     The hydraulic medium may be supplied to the supply gap particularly favorably when the gap communicates with a supply channel, wherein the supply channel is arranged, for example, on a side of the supply recess which is located opposite the sealing element and preferably opens into the piston bore with a supply opening which faces the piston skirt surface. 
     With respect to the design of the sealing element itself, no further details have so far been given. 
     One advantageous solution provides, for example, for the sealing element to have an annular base member. 
     Furthermore, the sealing element is preferably designed such that a supporting lip which is located radially outwards extends from the annular base member in the direction of the cylinder chamber. 
     Furthermore, the seal in relation to the piston skirt surface is preferably brought about in that a sealing lip which is located radially inwards extends from the base member in the direction of the cylinder chamber and abuts on the piston skirt surface. 
     In order to achieve a good seal, the sealing lip which is located radially inwards is preferably of a flexible design. 
     In this respect, the sealing lip is preferably designed such that it abuts on the piston skirt surface radially inwards with pretensioning. 
     With respect to the arrangement of the afterflow space in the case of a sealing element designed in such a manner, no further details have so far been given. 
     It is particularly advantageous when the afterflow space arranged between the sealing element and the piston surface extends in the area of the base member of the sealing element. 
     Furthermore, it is favorable when the afterflow space extends as far as the sealing lip. 
     Furthermore, with respect to the sealing lip only its sealing function relative to the piston skirt surface has been mentioned. 
     In order make a fast follow-up flow of hydraulic medium from the afterflow space into the cylinder chamber possible in the case of any extreme underpressure in the cylinder chamber, it is preferably provided for the sealing lip to have such a pretensioning that it allows hydraulic medium to flow through from the afterflow space to the cylinder chamber by at least partially lifting away from the piston skirt surface in the case of any underpressure in the cylinder chamber. 
     The piston is designed, in particular, such that it has a piston skirt surface which is cylindrical in relation to a central axis and extends with a constant cross section over the entire area of the piston which can be touched by the seal in all the positions of the piston. 
     With respect to the individual positions of the piston, no further details have so far been given. 
     One advantageous solution provides, for example, for the piston to be positioned in a closed position with the afterflow openings at such a distance from the pressure equalization position that the afterflow openings are covered by the sealing surface of the inner sealing lip and, therefore, no more equalization of pressure between the cylinder chamber and the afterflow space can take place. 
     Furthermore, it is advantageously provided for the piston to be displaced, in a pressure position, to such an extent in the direction of the cylinder chamber that the at least one afterflow opening is located on a side of the sealing lip facing the cylinder chamber and so, as a result, no equalization of pressure whatsoever can take place via the afterflow opening but rather the sealing lip, by resting on the unbroken piston skirt surface, abuts sealingly on it. 
     It is particularly favorable when a gap is present between an inner surface of the cylinder chamber and the piston skirt surface when the piston is in the pressure position and separates an annular chamber of the cylinder chamber, which adjoins the sealing element, from a variable volume area of the cylinder chamber. 
     Such a gap has the advantage that pressure pulsations which can propagate as far as into the variable volume area of the cylinder chamber merely have an effect in the annular chamber of the cylinder chamber which is damped or even better essentially not as fluctuations in pressure and, therefore, the sealing element can be protected from such pulsating fluctuations in pressure in the pressure position. 
     Such pressure pulsations can occur in clutch or unregulated brake systems; such pressure pulsations do, however, occur, in particular, in regulated brake systems, for example with ABS and/or ESC. 
     In this respect, it is particularly favorable when the gap between the wall surface of the cylinder chamber and the piston skirt surface is minimal in all the positions of the piston between the pressure position and the closed position. 
     The gap has a radial gap measurement which is, for example, in a range of 0.01 mm to 0.5 mm and a gap length extending parallel to a direction of movement of the piston of 0.1 mm to 10 mm. 
     Alternatively or in addition to the embodiments of the solution according to the invention described above, the object specified at the outset is also accomplished in accordance with the invention by a master cylinder of the type described at the outset in that the sealing element can be inserted into the cylinder housing from an outer opening thereof. 
     The advantage of this solution is to be seen in the fact that, as a result, the installation of the sealing element is made easier, in particular the installation of a sealing element according to one or several of the preceding features. 
     In this respect, it is particularly advantageous when the cylinder housing has a bore which lies adjacent to the outer opening, reaches as far as a base of a receiving groove for the sealing element and through which the sealing element can be inserted. 
     Such a bore which reaches as far as the base of the groove has the advantage that, as a result, it is not necessary to deform the sealing element to any great extent during its insertion into the housing but rather the sealing element can be inserted into the housing essentially undeformed. 
     In this respect, it is even more advantageous when the bore which lies adjacent to the outer opening has at least one cross sectional surface which corresponds to a cross sectional surface of the base of the groove and so the cross sectional surface of the bore either has the same cross section as the base of the groove or is larger and, therefore, the sealing element can be inserted into the groove in a particularly easy manner. 
     One expedient solution provides for an insert which fixes the sealing element in place to be insertable into the cylinder housing. Such an insert has the advantage that it is possible, as a result, to fix the sealing element in place. 
     The insert is preferably designed such that it forms a groove wall in the state inserted into the cylinder housing and, therefore, fixes the sealing element in place in the receiving groove. 
     In this respect, it is expediently provided for an additional groove wall which is not formed by the insert to be provided in the cylinder housing, this wall preferably being located opposite the groove wall formed by the insert. 
     Furthermore, the base of the groove is preferably provided in the cylinder housing. 
     A master cylinder according to the invention can, in principle, be used for any type of clutch, actuating and brake system. 
     Special advantages do, however, result when the master cylinder interacts with a regulated clutch, actuating or brake system. 
     Furthermore, the master cylinder can preferably be actuated by hand or by foot. 
     In addition, the invention relates to a sealing element which, in accordance with the invention, is designed in accordance with one or several of the preceding features. 
     Additional features and advantages of the invention are the subject matter of the following description as well as the drawings illustrating several embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a longitudinal section through a first embodiment of a master cylinder according to the invention in the pressure equalization position; 
         FIG. 2  shows a sectional enlarged illustration of the longitudinal section according to  FIG. 1  in the area of a sealing element; 
         FIG. 3  shows a perspective illustration of a sealing element from the side of the cylinder chamber; 
         FIG. 4  shows a perspective illustration of the sealing element of a supply gap; 
         FIG. 5  shows a longitudinal section through the sealing element according to the invention in the disassembled state; 
         FIG. 6  shows an even more enlarged illustration of the longitudinal section according to  FIG. 2  in the area of the sealing element and the supply gap; 
         FIG. 7  shows an illustration similar to  FIG. 1  of the master cylinder in the closed position; 
         FIG. 8  shows an illustration similar to  FIG. 2  of the master cylinder in the closed position; 
         FIG. 9  shows an illustration similar to  FIG. 1  of the master cylinder in the pressure position; 
         FIG. 10  shows an illustration similar to  FIG. 2  of the master cylinder in the pressure position; 
         FIG. 11  shows a section similar to  FIG. 2  through the cylinder housing of a second embodiment; 
         FIG. 12  shows a section along line  12 - 12  in  FIG. 11 ; 
         FIG. 13  shows an illustration similar to  FIG. 1  of a third embodiment of a master cylinder according to the invention; 
         FIG. 14  shows an illustration similar to  FIG. 1  of a fourth embodiment; 
         FIG. 15  shows an illustration similar to  FIG. 1  of a fifth embodiment; 
         FIG. 16  shows a perspective illustration of a slide insert of the fifth embodiment; 
         FIG. 17  shows an illustration of the slide insert according to  FIG. 14  in a plan view in the direction of arrow X in  FIG. 14  and 
         FIG. 18  shows a longitudinal section similar to  FIG. 5  in a sixth embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A first embodiment of a master cylinder  10  according to the invention, which is illustrated in  FIGS. 1 and 2 , in particular a master cylinder for a motor vehicle with a regulated brake system, comprises a cylinder housing which is designated as a whole as  12  and in which a piston bore which is designated as a whole as  14  is provided. 
     A piston designated as a whole as  16  is arranged in the piston bore  14 , wherein the piston  16  subjects a hydraulic medium to pressure which is arranged in a cylinder chamber  18  limited by the piston  16  and the piston bore  14 . 
     For this purpose, a hydraulic connection  20  is provided in the housing  12 , this connection opening into the cylinder chamber  18  on a side facing away from the piston  16  and a hydraulic system  22  being connectable to the cylinder chamber  18  via this connection in order to act, for example, on a clutch or brake system, which is not illustrated in detail, with the hydraulic medium subject to pressure via the hydraulic system  22 . 
     The piston bore  14  is preferably configured as a stepped bore and has in the area of the cylinder chamber  18  an inner bore section  24  which has a reduced diameter in relation to an outer bore section  26 , wherein the outer bore section  26  merges into the inner bore section  24  with a step  28 . The outer bore section  26  extends as far as an outer opening  30 , via which the piston  16  can be introduced into the piston bore  14 . 
     The piston  16  has, for its part, an inner piston section  36  which is adapted to the inner bore section  24  with respect to its diameter and an outer piston section  38  which is guided in the outer bore section  26  and adapted to its diameter. 
     The piston  16  is acted upon altogether by a pressure spring which is designated as a whole as  40 , is supported, on the one hand, on an end area  42  of the inner bore section  24  and, on the other hand, on a supporting surface  44  which is provided in the piston  16  and limits a recess  48  which penetrates the inner piston section  36  like a blind-end bore from an inner end side  46  of the piston  16  and in which the pressure spring  40  is guided with its section penetrating the inner piston section  36 . 
     Therefore, the pressure spring  40  always acts on the piston  16  in the direction of a pressure equalization position which defines a maximum volume of the cylinder chamber  18  and in which the piston  16  abuts with an outer end side  52  on a stop element  54 , for example a stop ring which is fixed to the cylinder housing  12  in the area of the outer opening  30 . 
     For the purpose of sealing the cylinder chamber  18 , a sealing element designated as a whole as  60  is provided which is arranged in a receiving groove  62  of the housing  12  which extends radially outwards into a wall area of the housing  12 , which surrounds the bore section  24 , as far as a groove base  68  with groove walls  65  and  67 , proceeding from an inner surface  64  of the inner bore section  24 . 
     In this respect, the sealing element  60  is arranged such that it abuts sealingly on a piston skirt surface  70 , which is rotationally symmetric and cylindrical and has an invariable cross section over a length of the piston  16 , in a forward area  72  located close to the inner end side  46  when the piston is in the pressure equalization position. 
     The sealing element  60  comprises, for its part, as illustrated, in particular, in  FIGS. 2 to 5 , an annular base member  74 , proceeding from which a supporting lip  76  extends which is located radially outwards, wherein the supporting lip  76  as well as the base member  74  abut with outer surfaces  66  on the base  68  of the groove and are supported by it. 
     Furthermore, a sealing lip  78  extends from the base member  74  preferably at an acute angle in relation to the supporting lip  76 , this sealing lip abutting on the forward area  72  of the piston skirt surface  70  with a sealing surface  80  facing the piston  16  when the piston  16  is in the pressure equalization position and terminating sealingly with this piston skirt surface. 
     In order to illustrate the construction of the annular sealing element  60 , this is illustrated again in detail in  FIGS. 3 to 5 . 
     The supporting lip  76  and the sealing lip  78  preferably have a V-shaped alignment relative to one another so that they extend approximately at an angle γ proceeding from the base member  74  in the non-installed state, wherein in the installed state the supporting lip  76  is deformed in the direction of the sealing lip  78  as a result of its abutment on the base  68  of the groove and the sealing lip  78  is deformed in the direction of the supporting lip  76  as a result of its abutment on the piston skirt surface  70  and so an angle γ′ between the supporting lip  76  and the sealing lip  78  is smaller in the installed state than the angle γ in the non-installed state of the sealing element  60 . 
     When the sealing element  60  is installed, the base member  74  rises above the base  68  of the groove to such an extent that an afterflow space  84  is formed between the piston skirt surface  70  and the inner surface area  82  of the sealing element  60 , in particular of the base member  74 , facing away from the base  68  of the groove and facing the piston  16  and this afterflow space extends in the area of the receiving groove  62  from the groove wall  67 , which faces away from the cylinder chamber  18 , in the direction of the cylinder chamber  18  as far as an annular cavity  88  in the sealing element  60  which is comprised by the inner surface areas  82  and is arranged in a foot area of the supporting lip  76  and approximately between the foot area of the supporting lip  76  and the base member  74  on a side facing the piston skirt surface  70  and which extends in a ring shape around the piston  16  (cf.  FIG. 2 ). 
     Additional overflow channels  90 , which are comprised by the inner surface areas  82  and extend from the annular cavity  88  in the direction of the groove wall  67 , lead as well to this cavity  88 . 
     The overflow channels  90  serve the purpose of providing an increased flow cross section for hydraulic medium flowing to the cavity  88  in the area of the afterflow space  84 . 
     The hydraulic medium enters the afterflow space  84 , as illustrated in  FIG. 2  and  FIG. 6 , through a supply gap  92  which extends in a wall section  93  of the inner bore section  24  from the groove wall  67  in the direction away from the cylinder chamber  18  as far as a supply channel  94  which has a supply opening  96  facing the piston skirt surface  70  as well as an entry opening  98 , via which hydraulic medium provided to a hydraulic connection  100  without pressure can enter the supply channel  94 . 
     The hydraulic medium flows through the supply channel  94  and exits from it through its supply opening  96  which adjoins the piston skirt surface  70  and flows from this supply opening  96  via the supply gap  92  to the afterflow space  84 . 
     The supply gap  92  is preferably designed such that it widens towards the afterflow space  84  proceeding from an area adjoining the supply opening  96 , as illustrated in  FIG. 6 . 
     In order to have the possibility of being able to compensate for any underpressure occurring in the cylinder chamber  18  due to a loss of hydraulic medium in the hydraulic system  22  in the pressure equalization position of the piston  16  by way of hydraulic medium running on from the afterflow space, the piston  16  is provided in its inner piston section  36  with afterflow bores  102  which pass through a piston wall  104  located between the piston skirt surface  70  and the recess  48  and, therefore, offer the possibility of hydraulic medium being able to enter the cylinder chamber  18  from the afterflow space  84  through these afterflow bores  102 , wherein the hydraulic medium enters an inner space  106  of the inner piston section  36  which is encircled by the recess  48  and from there can then pass into the cylinder chamber  18 , as illustrated in  FIG. 1  and  FIG. 2 . 
     The afterflow bores  102 , which are preferably distributed over the circumference of the inner piston section  36 , are located in the equalization position of the piston  16  such that they open into the afterflow space  84  in the area of the cavity  88  of the sealing element  60  so that the hydraulic medium flowing in via the supply channel  94  enters the afterflow space  84  through the supply opening  96  and the supply gap  92 , flows from there through the overflow channels  90  to the cavity  88  and from there, since it is designed to extend around the entire piston  16 , can enter the afterflow bores  102  and flow into the inner space  106  of the piston  16 . 
     Furthermore, a channel  110  leads from the hydraulic connection  100  to a hydraulic space  112  which is located on a side of the inner sealing element  60  facing away from the cylinder chamber  18  and is filled with hydraulic medium so that no air can enter the cylinder chamber  18  from a side of the inner sealing element facing away from the cylinder chamber  18  but rather only an exchange of hydraulic medium takes place in the case of a slight leakage. 
     The hydraulic space  112  extends as far as the outer piston section  38 , with which the piston  16  is guided in the outer bore section  26 , wherein a circumferential groove  114  is provided in the outer piston section  38  and the outer sealing element  120  is seated in this groove, likewise abuts on the base of the groove  114  with a supporting lip  124  proceeding from a base member  122  and abuts with a sealing lip  126  on the inner surface  128  of the outer bore section  26  in order to provide a seal between the hydraulic space  112  and the surroundings with air which adjoin the outer end side  52  of the piston  16 . 
     The outer sealing element  120  can be moved with the piston  16  so that the sealing lip  126  slides along on the inner surface  128 , wherein the hydraulic space  112  will become smaller when the piston  16  is moved in the direction of the cylinder chamber  18  and will become larger with movement in the direction away from the cylinder chamber  18 . 
     In order to build up pressure in the cylinder chamber  18 , the piston  16  is acted upon in the direction of the cylinder chamber  18  so that it moves in the direction of the cylinder chamber  18 , as illustrated in  FIGS. 6 and 7 , wherein the afterflow bores  102  likewise migrate in the direction of the cylinder chamber  18  with the movement of the piston  16  and, in this respect, as illustrated in  FIGS. 7 and 8 , first reach a closed position, in which the sealing lip  78  closes the afterflow bores  102  with its sealing surface  80  so that once this closed position is reached the afterflow space  84  and the cylinder chamber  18  are separated from one another and an equalization of pressure as a result of hydraulic medium flowing on from the afterflow space is no longer brought about. 
     A further movement of the piston  16  in the direction of the cylinder chamber  18  leads to a further displacement of the afterflow bores  102  beneath the sealing lip  78  into a pressure position which is illustrated in  FIGS. 9 and 10  and in which the afterflow bores  102  are located such that they open into an annular chamber  130  which is part of the cylinder chamber  18  and is located in the area of the annular groove  62  between a groove wall  132  located opposite the groove wall  86 , the sealing lip  89  and the supporting lip  76 , namely on a side of the sealing element  60  facing the cylinder chamber  18 , wherein the annular chamber  130  is separated from a variable volume area  138  of the cylinder chamber  18  by a gap  140  forming between the piston skirt surface  70  and the inner surface  64  in the pressure position of the piston  16 . 
     During further action on the piston  16 , the afterflow bores  102  migrate into a position, in which they are covered by the inner surface  64  in the area of the gap  140  and are, therefore, no longer functionable. 
     When the action on the piston  16  decreases, this migrates again in the direction of the pressure equalization position on account of the effect of the force of the pressure spring  40 , wherein the afterflow bores  102  again adjoin the annular chamber  130 , first of all, and then run through under the sealing surface  80  of the sealing lip  78  until they are again in a pressure equalization position which is in communication with the afterflow chamber  84  and in which the afterflow bores  102  provide a connection between the afterflow space  84  and the cylinder chamber  18  in order to allow hydraulic medium to flow on into the cylinder chamber  18  for the purpose of equalizing pressure. 
     In the case where a considerable underpressure results in the cylinder chamber  18 , the inner sealing element  60  is designed such that the sealing lip  78  has the possibility of lifting away from the piston skirt surface  70  or at least partially lifting away so that a larger cross section is available for hydraulic medium to flow on into the cylinder chamber  18  from the afterflow space  84  than is the case through the afterflow bores  102 . As a result, it is possible to also compensate quickly for large hydraulic losses in the cylinder chamber  18  and equalize any underpressure in the cylinder chamber resulting during the movement of the piston  16  from the pressure position in the direction of the pressure equalization position. 
     In a second embodiment, illustrated in  FIGS. 11 and 12 , of a master cylinder  10 ′ according to the invention, those parts which are identical to those of the first embodiment are given the same reference numerals and so in this respect reference can be made in full to the comments on the first embodiment. 
     In contrast to the first embodiment, supply cutouts  142  are provided in the wall section  93 ′ in the second embodiment instead of the gap  92  between the supply channel  94  and the afterflow space  84 , wherein the wall section  93 ′ extends as far as the step  28 ′ and so hydraulic medium can enter the afterflow space  84  from the side of the hydraulic space  112 . 
     In this respect, several supply cutouts  142  are preferably arranged around the piston  16  in the wall section  93 ′ and, proceeding from the step  28 , they have an ever decreasing depth with increasing extension in the direction of the cylinder chamber  18  proceeding from the cylindrical inner surface  64  and so the depth in the area of the groove wall  67  is minimal and, consequently, the sealing element  60  cannot be pressed into the supply cutouts  142 . 
     In a third embodiment of a master cylinder  10 ″ according to the invention, illustrated in  FIG. 13 , those parts which are identical to those of the preceding embodiments are given the same reference numerals and so with respect to their description reference can be made in full to the comments on the first embodiment. 
     In contrast to the first embodiment, the receiving groove  62  is designed such that the groove wall  67  as well as the groove base  68  are formed in the housing  12  but the base  68  of the groove is a section of an insert bore  150  which leads from the groove wall  86  as far as an outer opening  152 . 
     An insert designated as a whole as  160  can be inserted into this insert bore  150  and has, for its part, the groove wall  65  and forms with a sleeve member  162  the inner bore section  24  which surrounds the cylinder chamber  18  and into which the piston  16  can be moved with the inner piston section  36  for the purpose of taking up the pressure position. 
     Furthermore, the insert also accommodates the hydraulic connection  20  which opens into the cylinder chamber  18  on a side located opposite the piston  16 . 
     The insert member  160  offers the possibility of inserting the sealing element  60  into the housing  12  in a simple manner via the opening  152  and of fixing it in place between the groove wall  86  and the groove wall  132  after insertion of the insert  160 . 
     In a fourth embodiment of a master cylinder  10 ′ according to the invention, illustrated in  FIG. 14 , those parts which are identical to those of the preceding embodiments are given the same reference numerals and so with respect to the description of the remaining features reference can be made in full to the comments on the preceding embodiments. 
     In contrast to the preceding embodiments, the receiving groove  62 ″ is designed such that the groove wall  65  and the groove base  68  are formed in the housing  12  but the groove wall  67  is formed by an end surface  172  of a sleeve-like insert designated as a whole as  170 , the end surface being provided on an end flange  171 . 
     The insert  170  is seated in an insert bore  180  which extends into the housing  12  from the outer opening  30 ′″ on the side of the housing facing away from the cylinder chamber  18  and can be pushed into this insert bore  180  from the side of the opening  30 ′″ and can be fixed in place in it in order to facilitate installation of the sealing element  60  via the outer opening  30 ′″. 
     The insert  170  is, for its part, provided in the area of the end flange  171  with a cylindrical recess  174 , in which the piston  16  is guided with its outer piston section  38  in a comparable manner to that of the first embodiment in the wall section  93  of the inner bore section  24 . 
     Furthermore, the piston  16  is guided in a guide recess  176  of the insert  170  in the area of the its piston skirt surface  70 , namely adjoining the end surface  172 , so that the hydraulic space  112  is sealed in relation to the cylinder chamber  18  by the sealing element  60  within the insert  170  as in the first and second embodiments. 
     Moreover, the piston  16  is also sealed relative to the cylindrical recess  174  by means of the sealing element  120  in the same way as a seal is brought about in relation to the outer bore section  26  in the first and second embodiments. 
     In order to avoid any leakage between the insert  170  and the insert bore  180 , the insert  170  is also sealed relative to the insert bore  180  with a circumferential seal  182 . 
     The supply of hydraulic medium to the hydraulic space  112  is brought about via the channel  110  and openings  186  provided in a wall  184  of the insert  170 , wherein to allow hydraulic medium to run into the cavity  88  the guide recess  176  is likewise provided with a supply gap  92 ′ which communicates with the afterflow space  84  in the same way as that already described in conjunction with the first embodiment. 
     In the fourth embodiment, the sealing element  60  can, therefore, be pushed first of all via the outer opening  30 ′″ and the insert bore  180  as far as the base  68  of the groove and then be fixed in place in the completed groove  62 ′″ as a result of insertion of the insert  170  which forms the groove wall  67  so that the sealing element  60  can develop the same effect as in the preceding embodiments. 
     In a fifth embodiment of a master cylinder  10 ″″ according to the invention, illustrated in  FIGS. 15 to 17 , those parts which are identical to those of the preceding embodiments are given the same reference numerals and so with respect to the description of the remaining features reference can be made in full to the comments on these embodiments. 
     In contrast to the preceding embodiments, the receiving groove  62 ″″ is limited on its side facing away from the cylinder chamber  18  in that the groove wall  67  is formed by a slide insert member  191  of a slide insert  190  which can be pushed into a slide insert receptacle  192  in the cylinder housing  12  via the hydraulic inlet  100  transversely to the direction of movement of the piston  16 . 
     The slide insert receptacle  192  is preferably designed as a recess which extends even deeper into the cylinder housing  12  proceeding from the inner surfaces  128  of the outer bore section  26  in order to accommodate the slide insert  190  in the area of its edge regions  194 . 
     The slide insert  190  preferably forms the groove wall  67  by means of a side surface  196  which faces the cylinder chamber  20  and lies adjacent to a guide recess  198  in the slide insert  190 . 
     The guide recess  198  has the piston  16  passing through it when the slide insert  190  is installed in the cylinder housing  12  and guides the piston  16  in the area of the piston skirt surface  70  with a guide surface  199  at least in its pressure equalization position. 
     A supply of hydraulic medium via the hydraulic connection  100  is brought about by means of a channel  200  which is provided in the slide insert  190  and leads from a channel entry  204  to a channel exit  206 , which is located in the guide recess  198 , in an area  202  of the slide insert member  191  facing the hydraulic connection  100 , wherein the channel exit  206  is preferably located in one of several supply cutouts  208  which all serve to supply hydraulic medium from the hydraulic space  112  to the afterflow space  84  and the cavity  88 —as described in conjunction with the second embodiment—and are preferably arranged around the piston  16 . 
     The supply cutouts  208  extend from the guide surface  199  in the direction of the afterflow space  84  and increasingly less deeply into the slide insert member  191  and have the same function as the supply cutouts  142  of the second embodiment. 
     In the fifth embodiment, as well, the installation of the sealing element  60  is made easier since this can be inserted via an outer opening  30  and via the outer bore  26 , which lies adjacent to the opening and the cylindrical inner surface  128  of which preferably merges into the base  68  of the groove and thereby has the same cross sectional surface area as or a larger cross sectional surface area than the base  68  of the groove, as far as the base  68  of the groove so that a subsequent insertion of the slide insert  190  into the slide insert receptacle  192  secures the sealing element  60  in the receiving groove  62 . 
     The master cylinder  10 ″″ according to the fifth embodiment is then completed further by insertion of the piston  16  via the outer opening  30 . 
     In a sixth embodiment of a master cylinder according to the invention, illustrated in  FIG. 18 , the sealing element  60 ′ is provided between the cavity  88  and a ridge  210  of the sealing surface  80  with a channel  212  which limits the sealing surface  80  of the sealing lip  78  which rests on the piston skirt surface  70  to an area between the ridge  210  and the channel  212  and, therefore, improves the sliding of the sealing lip  78  on the piston skirt surface  70 . 
     Moreover, the channel  212  is dimensioned such that it holds hydraulic medium and acts in the manner of a lubricant pocket in order to ensure an easy sliding of the area of the sealing lip  78  located between the ridge  210  and the channel  212 . 
     As for the rest, all the additional parts are identical to those of one of the preceding embodiments and so reference can be made in full to the comments in conjunction with these embodiments.