Abstract:
A brake hose includes two reinforcing layers in a rubber base. The brake hose comprises an inner tube rubber layer having a flow path for flowing fluid, a lower yarn layer formed by braiding first yarns around the inner tube rubber layer and an upper yarn layer formed by braiding second yarns around the lower yarn layer and a cover rubber layer covering the upper yarn layer. A lower yarn layer duty LD given by the equation, LD(%)=(LRP/PRP)×100, has a value of 50-65%, 
     where LRP denotes an inner pressure at which a brake hose without the upper yarn layer and the cover would burst, and PRP denotes an inner pressure at which the brake hose would burst.

Description:
This application claims the benefit of and priority from Japanese Applications No. 2001-360375 filed Nov. 27, 2001 and No. 2001-360379 filed Nov. 27, 2001, the content of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention pertains to a brake hose having two reinforcing yarn layers including a lower yarn layer and an upper yarn layer in a rubber base. 
     2. Description of the Related Art 
     A brake hose known in the conventional art is shown in FIG. 16 (JP 06-201076A). FIG. 16 is a cross-section of the main components of a conventional brake hose  200 . Because the brake hose  200  must have high resistance against brake fluid pressure, it is formed from several layers of rubber and fiber yarn. The brake hose  200  comprises an inner tube rubber layer  202 , a lower yarn layer  204 , an intermediate rubber layer  206 , an outer yarn layer  208 , and a cover rubber layer  210 . 
     The brake hose  200  is required to meet a higher pressure resistance standard than a coolant system hose or a fuel system hose. As fluid temperatures and pressure levels of automobile system have increased in recent years, the demand for higher pressure resistance has increased as well. 
     The pressure from the pressure fluid flowing in the flow path  201  inside the brake hose  200  is transmitted from the inner circumference area of the brake hose  200  to the outer circumference area thereof. In other words, the pressure is transmitted to the inner tube rubber layer  202 , the lower yarn layer  204 , the intermediate rubber layer  206 , the upper yarn layer  208  and the cover rubber layer  210 , causing each layer to expand. Each layer has a binding force that operates against the pressure exerted by the pressure fluid and inhibits expansion of such layer. The inner tube rubber layer  202 , the intermediate rubber layer  206  and the cover rubber layer  210  are highly elastic, and are responsible for no more than 10% of the total binding force, while the majority of the binding force is possessed by the lower yarn layer  204  and the upper yarn layer  208 . Consequently, increasing the binding force of the lower yarn layer  204  and the upper yarn layer  208  increases the durability and expansion resistance (i.e., resistance to cubical expansion) provided by the brake hose  200 . As a result, increasing both of these characteristics by changing the type of yarn material used and the braiding method of the yarn layers has been examined. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a brake hose that offers increased durability and expansion resistance. 
     In accordance with one embodiment of the present invention, a brake hose includes two reinforcing layers in a rubber base. The brake hose comprises an inner tube rubber layer having a flow path for flowing fluid, a lower yarn layer formed by braiding first yarns around the inner tube rubber layer and an upper yarn layer formed by braiding second yarns around the lower yarn layer and a cover rubber layer covering the upper yarn layer. A lower yarn layer duty LD given by the equation, LD(%)=(LRP/PRP)×100, has a value of 50-65%, LRP denotes an inner pressure at which a brake hose without the upper yarn layer and the cover would burst, and PRP denotes an inner pressure at which the brake hose would burst. 
     In the brake hose pertaining to the present invention, the pressure exerted by the pressure fluid flowing in the flow path is transmitted from the inner circumference area of the brake hose to the outer circumference area thereof, i.e., from the interior of the rubber base to the lower yarn layer and the upper yarn layer, causing each such layer to expand. A binding force that restricts the expansion of the brake hose in resistance to the pressure from the pressure fluid is generated. The rubber base is responsible for no more than 10% of the total binding force due to its high elasticity, while the majority of the binding force is exerted by the lower yarn layer and the upper yarn layer. 
     Because the fluid pressure transmitted in this fashion travels from the inner circumference area to the outer circumference area of the brake hose in a radial fashion, the fluid pressure diminishes per unit area as it travels to the outer circumference area, and the lower yarn layer in the inner circumference area of the brake hose receives a larger amount of expansion force than the upper yarn layer. As a result, where the lower yarn layer and the upper yarn layer are formed from yarn made of the same material, the yarn of the lower yarn layer receives a greater tensile force than the yarn of the upper yarn layer. This means that even where the first yarns bursts after receiving a large amount of tensile force, there is still some degree of margin or leeway before the second yarns bursts. In view of this fact, the percentage burden assumed by the lower yarn layer is set at 50-65% of the total burden. In other words, the percentage burden assumed by the second yarns is set to a value larger than in a conventional brake hose, while the burden assumed by the first yarns is reduced. Consequently, the burden on each individual strand of yarn becomes smaller, and the ultimate rupturing pressure that may be applied to the brake hose can be increased. 
     The percentage burden assumed by the lower yarn layer is set to 50-65% because the first yarns and second yarns must satisfy prescribed levels of tensile strength and elongation. Therefore, a material such as vinylon, polyethylene terephthalate, polyethylene naphthalate or rayon is used, for example, because it would be difficult as a practical matter to set the percentage burden to be assumed by the lower yarn layer at less than 50% using these yarns, while if the lower yarn layer percentage burden were to exceed 65%, the higher rupturing pressure that comprises one characteristic of the present invention could not be obtained. 
     It is preferred that the second yarns of the upper yarn layer has a lower elongation than the first yarns of the lower yarn layer. Because the upper layer yarn is less subject to elongation when subjected to tensile force, it can handle a large amount of force up to the point at which the lower yarn layer expands and bursts. 
     For example, yarn having a tensile strength of 8.5 g per decitex and a elongation of 3.0±1% at a tensile load of 2.7 g can be used for the second yarns, and yarn having a tensile strength of 6.5 g per decitex and a elongation of 3.5±1% at a tensile load of 2.7 g can be used for the first yarns. A decitex is a unit of measurement that expresses the weight( 2 ) of a fiber relative to its length, and is equal to one gram per 10,000 meters of yarn. 
     The brake hose according to another aspect of the present invention comprises an inner tube rubber layer that has a flow path in which a pressure fluid flows and is formed from a rubber material, a lower yarn layer formed via braiding of first yarns around this inner tube rubber layer, an upper yarn layer that is formed via braiding of second yarns around this lower yarn layer, and a cover rubber layer that is formed around this upper yarn layer. The lower yarn layer is formed from first yarns strands that have on the surface thereof an adhesive thin film formed via RFL processing and a rubber thin film composed of EPDM that adheres to the adhesive thin film and the inner tube rubber layer, such layers formed in a sequential order. EPDM refers to ethylene-a-olefin-unconjugated diene copolymer (propylene as a-olefin). 
     The above-mentioned first yarns comprises a filament bundle composed of bundled filament threads, on each of which is formed an undercoat layer using an epoxy primer process. The above-mentioned adhesive thin film and rubber thin film are sequentially applied to the outer surface of each filament bundle. 
     The above-mentioned first yarns comprises a filament bundle composed of bundled filament threads. An undercoat is formed on the outer surface of each filament bundle using an epoxy primer process, and an adhesive thin film and rubber thin film are then applied over the undercoat. 
     In the brake hose pertaining to the present invention, the lower yarn layer and the upper yarn layer formed around the inner tube rubber layer form two reinforcing yarn layers inside the rubber base, and give the brake hose sufficient strength to withstand the high pressure of the pressure fluid flowing within the flow path. Furthermore, the first yarns constituting the lower yarn layer includes an adhesive thin film formed via RFL processing and a rubber thin film. The rubber thin film adheres to the inner tube rubber layer and prevents yarn displacement, increases the solidity of the lower yarn layer by causing the strands of the first yarns to adhere to each other at areas where they overlap, which prevents the inner tube rubber layer from expanding due to internal pressure, thereby limiting the amount of cubical expansion of the brake hose and improving the feel of the brake. The adhesive thin film formed via RFL processing is formed in order to cause the first yarns to adhere to the rubber thin film formed from EPDM. In RFL processing, an adhesive thin film that operates as an adhesive and is formed mainly from resorcinol-formaldehyde-latex resin and rubber latex is applied to the surface of each yarn strand. 
     In the brake hose according to another aspect of the present invention, because an intermediate rubber layer is not formed around the lower yarn layer, the process of forming the intermediate rubber layer can be omitted. As a result, when a manufacturing run of brake hoses is produced, the significant amount of floor space required for forming the intermediate rubber layer is no longer required. 
     In a preferred embodiment of the first yarns, a filament bundle is formed by bundling together several hundred filament threads, over each of which is formed an undercoat layer using an epoxy primer process, and then forming over the filament bundle a layer formed via RFL processing and an EPDM layer. The lower yarn layer is then formed by braiding the first yarns around the inner tube rubber layer. In this case, because the filament threads adhere strongly to each other due to the undercoat layer, the penetration of air or brake fluid between the filament threads can be prevented more effectively. 
     In another preferred embodiment of the first yarns, a filament bundle is formed by bundling together filament threads, an undercoat is formed on the outer surface of the filament bundle using an epoxy primer process, and an adhesive thin film and a rubber thin film are sequentially formed over the undercoat layer. In this case, because the epoxy primer process is not performed for each individual filament thread, and is instead carried out for the filament bundle as a whole, manufacturing efficiency can be improved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial cutaway perspective view of the brake hose pertaining to a first embodiment of the present invention; 
     FIG. 2 is a half-sectional view of the main components of the brake hose; 
     FIG. 3 shows a hose manufacturing apparatus; 
     FIG. 4 shows the types of yarn in the lower yarn layer and upper yarn layer and the physical property values therefor in an embodiment 1 and in comparison examples 1-3; 
     FIG. 5 is a graph showing the relationship between the percentage burden assumed by the lower yarn layer and hose durability; 
     FIG. 6 is a graph showing the relationship between the percentage burden assumed by the lower yarn layer and the amount of cubical expansion; 
     FIG. 7 is a partial cutaway perspective view of the brake hose pertaining to a second embodiment of the present invention; 
     FIG. 8 is a half-sectional view of the main components of the brake hose; 
     FIG. 9 shows an expanded cross-section of the lower yarn; 
     FIG. 10 shows the processes used to produce the lower yarn; 
     FIG. 11 is an expanded cross-section showing a part of the upper yarn; 
     FIG. 12 shows a hose manufacturing apparatus; 
     FIG. 13 is a graph showing the amount of cubical expansion in an embodiment and in a comparison example; 
     FIG. 14 is a graph showing pressure resistance in an embodiment and in a comparison example; 
     FIG. 15 is a cross-section showing the lower yarn pertaining to another embodiment of the present invention; and 
     FIG. 16 is a half-sectional view showing the brake hose of the prior art. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A. First embodiment 
     (1) Basic Construction of Brake Hose  10   
     FIG. 1 is a partial cutaway perspective view of a brake hose  10  pertaining to a first embodiment of the present invention. FIG. 2 is a partial cross-sectional view of the brake hose  10 . With reference to FIG.  1  and FIG. 2, the brake hose  10  is used in order to connect a master cylinder used to carry out hydraulic braking in an automobile not shown in the figures to a hydraulic device mounted near each tire, and comprises five layers in order to withstand the brake fluid pressure. The brake hose  10  includes an inner tube rubber layer  12  having a flow path  11 , a lower yarn layer  14 , an intermediate rubber layer  16 , an upper yarn layer  18 , and a cover rubber layer  20 . A mouthpiece  22  is fixed to an end of the brake hose  10  via caulking. 
     (2) Construction of Various Layers of the Brake Hose  10   
     In order to give the brake hose such properties as pressure resistance against the brake fluid pressure of up to 50 MPa, durability and expansion resistance, the materials, thickness and other parameters of each layer are to established. 
     (2)-1 Inner Tube Rubber Layer  12   
     Primarily in order to achieve oil resistance, the inner tube rubber layer  12  is made of ethylene-propylene-diene copolymer rubber (EPDM), styrene-butadiene rubber copolymer (SBR) or the like, and has an inner diameter of 3.0-3.4 mm and a thickness of 0.5-1.0 mm. 
     (2)-2 Lower Yarn Layer  14   
     The lower yarn layer  14  comprises a wound yarn including two or three strands of a fiber such as vinylon, polyethylene terephthalate, polyethylene napththalate or rayon, and is formed by braiding the yarn around the inner tube rubber layer  12  using a braid count of 20 count or 24 count. A braid count of 20 count or 24 count means that the yarn is drawn from bobbins located at 20 or 30 locations and braided around the inner tube rubber layer  12 , as described below. 
     Based on the relationship to the percentage burden borne by the upper yarn layer described below, it is preferred that the lower yarn of the lower yarn layer constitute a yarn having a tensile strength of 6.5 g or more per decitex and a elongation of 3.5±1% at a tensile load of 2.7 g. 
     (2)-3 Intermediate Rubber Layer  16   
     The intermediate rubber layer  16  is a layer intended to prevent displacement of the lower yarn layer  14  and the upper yarn layer  18 . The upper yarn layer  18  is formed by winding a sheet made from a rubber material around the lower yarn layer  14  or by applying rubber cement onto the lower yarn layer  14 . 
     As the rubber material for the sheet, EPDM, isobutylene-isoprene copolymer rubber (IIR) or natural rubber (NR) may be used. Using EPDM or IIR or a material comprising a mixture of the two permits a higher heat resistance because of the properties of such materials. 
     It is preferred that the Mooney viscosity of the intermediate rubber layer  16  be 10-40 Mv (minimum Mooney value) at 145° C. The Mooney viscosity is a value reflecting measurement of the viscosity of non-vulcanized rubber at 145° C. based on the K6300 test promulgated under JIS (Japanese Industrial Standards). The Mooney viscosity is set to the above range in order to ensure that the intermediate rubber layer  16  fills in the gaps between the strands of the lower yarn  15  and the strands of the upper yarn  19 , thereby preventing displacement of the lower yarn layer  14  and the upper yarn layer  18 . When a sheet material having a Mooney value of 10-40 Mv (minimum Mooney value) at 145° C. is used in order to increase the effect of the intermediate rubber layer  16 , the intermediate rubber layer  16  fills in the gaps between the strands of the lower yarn of the lower yarn layer  14  and the strands of the upper yarn of the upper yarn layer  18  when the upper fiber layer  19  is braided, thereby inhibiting displacement of the yarn of the lower yarn layer  14  and the upper yarn layer  18 . Therefore, when internal pressure is generated, there is minimal displacement of the yarn of the lower yarn layer  14  and the upper yarn layer  18 , and durability and cubical expansion resistance can be improved. The Mooney viscosity for the rubber material used for the intermediate rubber layer  16  may be adjusted by, for example, changing the type of carbon added to the rubber material. 
     It is preferred that the intermediate rubber layer  16  have a thickness of 0.1-0.25 mm. This is because if the thickness is less than 0.1 mm, the intermediate rubber layer  16  becomes too thin and cannot be braided to around the lower yarn layer  14 , while if the thickness exceeds 0.25 mm, the intermediate rubber layer is so thick that the intermediate rubber layer  16  functions as an elastic layer that permits displacement of the lower yarn layer  14 , and the displacement-inhibiting effect of the lower yarn layer is diminished. 
     Where rubber cement is used for the intermediate rubber layer  16 , the Mooney value of the rubber cement is close to zero, and the above thickness is obtained by applying the rubber cement in several coats (for example, in at least three coats). The rubber cement comprises a blend of IIR and EPDM dissolved in trichloroethane. 
     (2)-4 Upper Yarn Layer  18   
     The upper yarn layer  18  is formed by braiding around the intermediate rubber layer  16  the upper yarn  19  obtained by winding together two or three fiber threads of vinylon, polyethylene terephthalate, polyethylene napththalate or rayon, using a braid count of 20 count or 24 count. The upper yarn  19  is formed by bundling together 200-400 filament threads and braiding yarn obtained by winding together two or three such filament threads using a braid count of 20 count or 24 count. 
     For the upper yarn  19 , a yarn that has a lower elongation than the lower yarn  15 , such as a yarn having a tensile strength of 8.5 g per decitex and a elongation of 3.0±1% at a tensile load of 2.7 g, can be used. Yarns made from the same material but having a different tensile force can be manufactured by subjecting the yarn to an elongation and heating process in which the yarn is pulled while being heated. 
     (2)-5 Cover Rubber Layer  20   
     Mainly in order to achieve ozone resistance, the cover rubber layer  20  is made of a material such as EPDM or a blend of EPDM and CR. The cover rubber layer  20  has a thickness of 0.5-1.0 mm. 
     (3) Brake Hose  10  Manufacturing Method 
     The manufacturing method for the brake hose  10  will now be described. The brake hose  10  can be manufactured using public-domain methods, i.e., by carrying out a rubber extrusion process, a fiber yarn braiding process and a vulcanization process. 
     (3)-1 Hose Manufacturing Apparatus  30   
     FIG. 3 shows a hose manufacturing apparatus  30 . With reference to FIG. 3, the hose manufacturing apparatus  30  includes a first extruding device  31 , a first braiding device  32 , an intermediate sheet forming device  34 , a second braiding device  35  and a second extruding device  37 . The first extruding device  31  is a device that forms the inner tube rubber layer  12  by extruding a rubber material. The first braiding device  32  includes bobbin carriers (not shown in the figure) mounted to a drum  32   a , and forms the lower yarn layer  14  by braiding the lower yarn  15  around the extruded inner tube body  12 A while drawing the lower yarn  15  from the bobbin carriers. The intermediate sheet forming device  34  draws from a roller a sheet material  16 A used to form the intermediate rubber layer  16  around the lower yarn layer  14  braided by the first braiding device  32 . The second braiding device  35  has a construction essentially identical to that of the first braiding device  32 . The second braiding device  35  includes bobbin carriers (not shown in the figure) mounted to a drum  35   a , and forms the upper yarn layer  18  by braiding the upper yarn  19  around the intermediate rubber layer  16  while drawing the upper yarn  19  from the bobbin carriers. The second extruding device  37  forms the cover rubber layer  20  by extruding a rubber material and covering the rubber material over the upper yarn layer  18 . 
     (3)-2 Brake Hose  10  Manufacturing Process 
     The series of manufacturing steps by which the brake hose  10  is manufactured by the hose manufacturing apparatus  30  will now be described. First, the inner tube rubber layer  12  is formed via extrusion of a rubber material by the first extruding device  31 . During this process, a mandrel (not shown in the figure) is inserted inside the inner tube rubber layer  12 . Next, the lower yarn layer  14  is formed by drawing lower yarn  15  from the bobbins while the drum  32   a  of the first braiding device  32  rotates and braiding the lower yarn  15  around the extruded inner tube rubber layer  12 . During this process, in order to braid the lower yarn layer  14  to a braid count of  20 , for example, the lower yarn  15  is drawn from bobbins that are located at  20  locations and rotate in opposite directions. The intermediate rubber layer  20  is then formed by supplying the sheet material  16 A from the intermediate sheet forming device  34  over the lower yarn layer  14 . The upper yarn layer  18  is then braided around the intermediate rubber layer  16  by drawing upper yarn  19  from the bobbins while the drum  35   a  of the second braiding device  35  rotates. Finally, the cover rubber layer  20  is formed by extruding a rubber material from the second extruding device  37  over the upper yarn layer  18 . 
     The vulcanization process is then performed. Vulcanization is carried out for 15-60 minutes at 120-170° C. Due to the heating that occurs during the vulcanization process, the RFL-processed upper yarn layer  18  and lower yarn layer  14  adhere to the inner tube rubber layer  12 , the intermediate rubber layer  16  and the cover rubber layer  20 . Consequently, the brake hose  10  is integrally formed. 
     (4) Brake Hose Operation and Effect 
     (4)-1 Lower Yarn Layer Percentage Burden 
     Durability and expansion resistance (i.e., the amount of cubical expansion) in connection with changes in the percentage burden borne by the lower yarn layer were investigated. FIG. 4 shows the types of yarn in the lower yarn layer and upper yarn layer and the physical property values therefor in an embodiment and in comparison examples 1-3 created as test samples. Polyester (PET) fiber was used as the lower yarn, and the yarn used in the embodiment and the comparison example 1 had the same physical property values, while yarns having a different elongation and tensile strength were used in the comparison examples 2 and 3. Vinylon was used as the upper yarn in the embodiment and in the comparison examples 1 and 2, while polyester was used for the comparison example 3. Furthermore, the upper yarn used in the embodiment had a higher tensile strength and a lower elongation than the upper yarn used in the comparison examples. The other dimensions of the brake hose were as follows: the outer diameter was 10.5 mm, the length was 305 mm, the inner diameter of the inner tube rubber layer was 3.2 mm, the thickness of the inner tube rubber layer was 0.8 mm, and the thickness of the cover rubber layer was 0.8 mm. For the intermediate rubber layer, a sheet material made from 0.2 mm-thick EPDM was used. 
     The lower yarn layer duty was sought by (1) creating a brake hose, applying internal pressure and measuring the pressure when a burst occurred and deeming this pressure the product burst pressure, (2) measuring the pressure that caused burst when the cover rubber layer, the upper yarn layer and the intermediate rubber layer had been removed from the brake hose, and deeming this pressure the lower yarn layer burst pressure, and (3) seeking the lower yarn layer duty using the following formula: 
     
       
         Lower yarn layer duty (%)=(lower yarn layer burst pressure/product burst pressure)×100. 
       
     
     (4)-2 Durability Test 
     The durability test was performed via repeated pressure testing in which brake fluid was actually sent through the brake hose. In other words, under an ambient temperature of 120° C., brake fluid was injected at cycles of 0.3 Hz at a fluid pressure ranging from 0 MPa to 20 MPa, and the number of injections required for product burst was determined. The results are shown in FIG.  5 . In the graph of FIG. 5, the vertical axis indicates an index for the number of times fluid pressure applied, while the horizontal axis indicates the lower yarn layer duty. The burden percentage of less than 65% pertaining to the embodiment was compared with the burden percentages of 67%, 75% and 78% pertaining to the comparison examples 1, 2 and 3, respectively, from which it was determined that a burden percentage of under 65% offers improved durability. This is due to the fact that increases in the brake fluid pressure were borne by the upper yarn layer, which had a higher tensile strength and a lower elongation, whereby application of a large load on the lower yarn was avoided and the durability of the brake hose was improved. 
     (4)-3 Cubical Expansion Amount Test 
     In the cubical expansion amount test, the amount of cubical expansion was determined by measuring in accordance with JIS standard  2601  the change in the internal volume of a 305 mm-length of brake hose when oil pressure of 10.3 MPa was generated therein. The results are shown in FIG.  6 . FIG. 6 is a graph in which the vertical axis indicates the amount of cubical expansion, while the horizontal axis indicates the lower yarn burden percentage. As in the durability test, it was determined that the amount of cubical expansion falls when the lower yarn burden percentage is less than 65%. This is due to the fact that increases in brake fluid pressure were borne by the upper yarn layer having a higher tensile strength and a lower elongation, i.e., the resistance of the brake hose against the expansion force was increased by the upper yarn layer, whereby the amount of cubical expansion was reduced. 
     A. Second Embodiment 
     (1) Basic Construction of Brake Hose  110   
     FIG. 7 is a partial cutaway perspective view of the brake hose  110  pertaining to a second embodiment of the present invention, while FIG. 8 is a cross-sectional view of the main components of the brake hose  110 . With reference to FIGS. 7 and 8, the brake hose  110  includes an inner tube rubber layer  112  having a flow path  111 , a lower yarn layer  114  braided around the inner tube rubber layer  112 , an upper yarn layer  118  braided around the surface of the lower yarn layer  114 , and a cover rubber layer  120  that covers the surface of the upper yarn layer  118 . A mouthpiece  122  is fixed to one end of the brake hose  110  via caulking. 
     (2) Construction of Various Layers of Brake Hose  110   
     (2)-1 Inner Tube Rubber Layer  112   
     Primarily in order to achieve oil resistance, the inner tube rubber layer  112  is formed from ethylene-propylene-diene copolymer rubber (EPDM), styrene-butadiene rubber copolymer (SBR) or the like, and has an inner diameter of 3.0-3.4 mm and a thickness of 0.5-1.0 mm. 
     (2)-2 Lower Yarn Layer  114   
     The lower yarn layer  114  comprises a wound yarn composed of two or three strands of a fiber such as vinylon, polyethylene terephthalate, polyethylene napththalate or rayon, and is formed by coating the yarn with an adhesive and a rubber layer and braiding the yarn around the inner tube rubber layer  112  using a braid count of 20 count or 24 count. 
     FIG. 9 is an explanatory drawing showing an expanded cross-section of the lower yarn  115 . The lower yarn  115  comprises a filament bundle  115   b  composed of 250-400 bundled filament threads  115   a , an undercoat layer  115   c  that is applied to the outer surface of the filament bundle  115   b , an adhesive thin film  115   d  formed around the filament bundle  115   b , and a rubber thin film  115   e  formed around the adhesive thin film  115   d . In FIG. 9, the filament bundle  115   b  is represented by only a small number of filament threads  115   a  for simplification purposes. The undercoat layer  115   c  is formed from epoxy resin in order to ensure adherence between the filament threads  115   a  and the adhesive thin film  115   d . The adhesive thin film  115   d  is RFL-processed in order to ensure adherence between the undercoat layer  115   c  and the rubber thin film  115   e . The rubber thin film  115   e  is a rubber layer formed from EPDM and serves to ensure adherence between the adhesive thin film  115   d  and the inner tube rubber layer  112  and to prevent displacement or abrasion of the lower yarn  115 . 
     The lower yarn is manufactured according to the processes described below. FIG. 10 is an explanatory drawing describing the processes used to manufacture the lower yarn  115 . First, a hopper  123   a  is filled with polyester resin, filament threads are extruded by an extruder  123   b , and these filament threads are bundled together while passing through a cooler  123   c . This forms a filament bundle  115   b  composed of approximately 250-400 bundled filament threads  115   a . The filament bundle  115   b  then passes through an undercoating apparatus  123   d  and receives a coating of epoxy resin. The undercoating apparatus  123   d  forms an undercoat layer  115   c  comprising an adhesive undercoat to the outer surface of the filament bundle  115   b  by impregnating the filament bundle  115   b  with epoxy resin using nip rollers. The filament bundle  115   b  is then passed through elongation apparatuses  124   b  and  124   c  to elongate the filament bundle  115   b  in order to adjust the elongation thereof. 
     An RFL application process is then performed. The RFL application process forms an adhesive thin film  115   d  on the filament bundle  115   b  by impregnating the filament bundle  115   b  with an RFL adhesive solution using nip rollers after the filament bundle  115   b  has been dipped in an RFL tank  25   b  containing the RFL adhesive solution, and then drying the impregnated filament bundle  115   b  in a drying apparatus  125   c . The RFL adhesive solution is a mixture of an aqueous solution of initial concentrate of resorcinol formaldehyde and rubber latex. The aqueous solution of initial concentrate can be prepared by inducing a reaction between 1 mol of resorcinol and 0.75-0.8 mol of formaldehyde in a base catalyst at close to room temperature. A base substance such as sodium hydroxide or ammonium hydroxide is preferred as the base catalyst. Natural rubber latex or synthetic rubber latex may be used as the above rubber latex material. If synthetic rubber latex is used, such latex may consist of styrene-butadiene copolymer rubber latex, vinylpyridine-butadiene-styrene copolymer rubber latex or the like. 
     Next, an EPDM application process is performed. In the EPDM application process, a rubber thin film  115   e  is formed over the adhesive thin film  115   d  by impregnating the filament bundle  115   b  to which the adhesive thin film  115   d  was applied by running it through nip rollers after it has been dipped in the EPDM tank  26   b  containing non-vulcanized EPDM, and then drying it using a drying apparatus  126   c.    
     It is preferred that the amount of rubber thin film  115   e  equal 5-30% by weight of the total weight of the filament thread  115   a . This is because if the weight is less than 5%, the effect of preventing abrasion or displacement of the filament threads is not sufficiently obtained due to contact friction between the strands of the lower yarn  115   a , while if the weight exceeds 30%, elastic deformation due to the rubber layer becomes large, resulting in significant thread displacement. 
     (2)-3 Upper Yarn Layer  118   
     The upper yarn layer  118  shown in FIG. 8 is formed by braiding the upper yarn  119 . FIG. 11 is an expanded cross-section showing a part of the upper yarn  119 . The upper yarn  19  is formed by bundling together 250-400 adhesive threads  119   a . In other words, the adhesive threads  119   a  are made of polyethylene terephthalate, an undercoat layer  119   c  is formed by applying undercoating adhesive to the thread bundle, and an adhesive thin film  119   d  made of an RFL layer is formed over the undercoat layer  119   c , thereby creating the upper yarn  118 . The adhesive thin film  119   d  is an adhesive thin film layer that increases the adhesive force between the lower yarn layer  114  and the EPDM rubber of the cover rubber layer  120 . 
     (2)-5 Cover Rubber Layer  120   
     Mainly in order to achieve ozone resistance, the cover rubber layer  120  shown in FIG. 8 is made of a material such as EPDM or a blend of EPDM and CR. The cover rubber layer  120  has a thickness of 0.5-1.0 mm. 
     (3) Brake Hose  110  Manufacturing Method 
     The manufacturing method for the brake hose  110  will now be described. The brake hose  110  can be manufactured using public-domain methods, i.e., by carrying out a rubber extrusion process, a fiber yarn braiding process and a vulcanization process. 
     (3)-1 Hose Manufacturing Apparatus  130   
     FIG. 12 is an explanatory drawing to describe a hose manufacturing apparatus  130 . The hose manufacturing apparatus  130  includes a first extruding device  131 , a first braiding device  132 , a second braiding device  135  and a second extruding device  137 . The first extruding device  131  is a device that forms the inner tube rubber layer  112  by extruding a rubber material. The first braiding device  132  includes bobbin carriers mounted to a drum  132   a , and forms the lower yarn layer  114  by braiding the lower yarn  115  around the inner tube rubber layer  112  while drawing it from the bobbin carriers. The second braiding device  135  has a construction essentially identical to that of the first braiding device  132 . It includes bobbin carriers mounted to a drum  135   a , and forms the upper yarn layer  118  by braiding the upper yarn  119  around the lower yarn layer  114  while drawing it from the bobbin carriers. The second extruding device  137  forms the cover rubber layer  120  by extruding a rubber material and covering the upper yarn layer  118  therewith. 
     (3)-2 Brake Hose  110  Manufacturing Process 
     The series of manufacturing steps by which the brake hose  110  is manufactured by the hose manufacturing apparatus  130  will now be described. First, the inner tube rubber layer  112  is formed via extrusion of a rubber material by the first extruding device  131 . During this process, a mandrel (not shown in the figure) is inserted inside the inner tube rubber layer  112 . Next, the lower yarn layer  114  is formed by drawing lower yarn  115  from the bobbins while the drum  132   a  of the first braiding device  132  is rotating and braiding the lower yarn  115  around the extruded inner tube rubber layer  112 . During this process, in order to braid the lower yarn layer  114  to a braid count of 20, for example, the lower yarn  115  is drawn from bobbins that are located at 20 locations and rotate in opposite directions. The upper yarn layer  118  is then braided around the lower yarn layer  114  by drawing upper yarn  119  from the bobbins while the drum  135   a  of the second braiding device  135  is rotating. Finally, the cover rubber layer  120  is formed by extruding the rubber material used for the cover rubber layer  120  from the second extruding device  137  and forming it over the upper yarn layer  118 . 
     The vulcanization process is then performed under normal conditions. For example, vulcanization is carried out for 15-60 minutes at 145-165° C. As a result of the vulcanization process, the inner tube rubber layer  112  and the rubber thin film  115 e formed on the lower yarn  115  become bonded through normal vulcanization adhesion. In other words, the lower yarn  115  of the lower yarn layer  114  adheres to the rubber thin film  115   e  due to the heat applied during vulcanization via the adhesive thin film  115   d  formed by the RFL process. Consequently, the brake hose  110  is formed as a single unit. 
     (4) Brake Hose  110  Operation and Effect 
     (4)-1 In the brake hose  110  described above, because the lower yarn layer  114  and the upper yarn layer  118  form two separate reinforcing layers inside the rubber base, the brake hose  110  can be made strong enough to withstand the high pressure exerted by the pressure fluid flowing in the flow path  111 . 
     (4)-2 Because the brake hose  110  does not have an intermediate rubber layer formed around the lower yarn layer  114  like the brake hose of the conventional art, the process of forming the intermediate rubber layer can be omitted. As a result, the significant amount of floor space required for forming the intermediate rubber layer is no longer required. 
     (4)-3 Because the lower yarn  115  comprises approximately 250-400 filament threads  115   a  bound into a bundle, undergoes RFL processing via the undercoat layer  115   c , and furthermore has a rubber thin film  115   e , it adheres strongly to the inner tube rubber layer  112  via the rubber thin film  115   e , and there is no displacement of the strands of the lower yarn  115 . Moreover, because the unitary construction of the lower yarn layer  114  is promoted by the mutual adhesion of the strands of the lower yarn  115  caused by the vulcanization adhesion effect of the rubber thin film  115   e , expansion of the inner tube rubber layer  112  from the pressure of the pressure fluid in the flow path is inhibited. In other words, the braking feel is improved due to the minimization of the amount of cubical expansion of the rake hose  110 . Furthermore, the rubber thin film  115   e  strongly bonds to the epoxy fiber threads due to the RFL processing and the undercoat layer  119   c.    
     Testing to determine durability and the amount of cubical expansion of the brake hose was then performed. The results are shown in FIGS. 13 and 14. The brake hoses used in the embodiment and in the comparison example had an outer diameter of 10.5 mm, a length of 305 mm, an inner tube rubber layer inner diameter of 3.2 mm, an inner tube rubber layer thickness of 0.8 mm, and a cover rubber layer thickness of 0.8 mm. The lower yarn was made of polyethylene terephthalate fiber, while the upper yarn was made of vinylon fiber. 
     The cubical expansion test investigated the amount of volume fluctuation when fluid pressure of 10.5 MPa was applied. The durability test was carried out as follows: at room temperature, the brake hose was repeatedly injected with brake fluid at cycles of 0.3 Hz at a fluid pressure ranging from 0 MPa to 20 MPa, and the number of injections required for product burst was determined. 
     From the results of the tests, it was determined that the embodiment offered performance that was equal or superior to that provided by the comparison example representing the conventional art. 
     FIG. 15 is an explanatory drawing to describe the lower yarn  115 B pertaining to another embodiment of the present invention via a cross-section thereof. The lower yarn  115 B is characterized in that rather than having an undercoat layer formed over the outer surface of the filament bundle  115 B b , an undercoat layer  115 B c  is formed over each individual filament thread  115 B a . In other words, the lower yarn  115 B comprises a filament bundle  115 B b  composed of 250-400 filament threads  115 B a  that are bundled together. The undercoat layer  115 B c  is applied to the outer surface of each individual filament thread  115 B a . The filament bundle  115 B b  is formed by bundling together these filament threads  115 B a , and an adhesive thin film  115 B d  and a rubber thin film  115 B e  are then sequentially formed over the filament bundle  115 B b . When the lower yarn  115 B is manufactured, the undercoat layer  115 B c  is formed immediately after the filament threads  115 B a  are extruded, the filament bundle  115 B b  is formed by bundling these filament threads  115 B a , and the adhesive thin film  115 B d  formed via RFL processing and the rubber thin film  115 B e  are thereafter formed sequentially over the coated filament bundle  115 B b . This lower yarn  115 B provides the same operation and effect as the embodiments described above. 
     This invention is not limited to the above embodiments, and may be implemented in various ways within the essential scope of the invention. For example, the variation described below may be applied. 
     In the above embodiments, the rubber thin film was formed only over the yarn of the lower yarn layer, but it is acceptable if a rubber skin layer is formed over the upper yarn in the same fashion. 
     The foregoing detailed description of the invention has been provided for the purpose of explaining the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use contemplated. The foregoing detailed description is not intended to be exhaustive or to limit the invention to the precise embodiments disclosed. Modifications and equivalents will be apparent to practitioners skilled in this art and are encompassed within the spirit and scope of the appended claims.