Cap for reserve tanks
FIELD OF THE INVENTION AND RELATED ART STATEMENT 
     The present invention relates to a cap for reserve tanks, and more particularly to a cap for liquid reserve tanks for use in the master cylinder and the like. 
     In a liquid reserve tank, a reservoir chamber is opened to an atmosphere via a vent passage defined in a cap and the like, since it is required for the reservoir chamber to be constantly maintained at an atmospheric pressure. 
     Especially in an oil reservoir for a device mounted on vehicles, for example, a master cylinder, the vent passage is formed in a labyrinthine configuration, since there is a possibility that an oil contained in the reservoir chamber may possibly be leaked out to a surrounding atmosphere through the above-described vent passage as a result of vibration and the like of the vehicle. 
     FIG. 3 illustrates the oil reservoir used in such master cylinder. 
     The cap 2 of reserve tanks 1 is provided at its upper member 3 with an annular rib 4, and at its lower member 5 with first and second annular ribs 6 and 7. These upper and lower members 3 and 5 are integrated together in such a manner that the annular rib 4 may face toward the interior of the annular rib 6, while a plate 8 is contained inside the annular rib 6 and is retained in the rib 6 at the edge of the rib 4. The upper member 3 is provided at the edge of the rib 4 with a groove 4a, thereas the lower member 5 is provided at the edges of the ribs 6 and 7 with grooves 6a and 7a. In turn, a hole 5a is defined through the bottom wall of the rib 6 and a hole 8a is defined through the plate 8. 
     These ribs 4, 6 and 7, and the plate 8 serve to form a labyrinth between the upper and lower members 3 and 5, in combination with the grooves 4a, 6a, 7a, and holes 8a and 5a etc., 
     In this type of cap 2, a sealing member 10 which has been engaged with the reserve tank 9 and arranged along the circumferential surface of the lower member 5 is brought into abutment against the inner circumferential surface of the reserve tank 9 so as to create a seal between the cap 2 and the reserve tank body 9. 
     Accordingly, in the oil reserve tank 1, a reservoir 
     chamber 11 is opened to an atmosphere through the holes 5a and 8a, and grooves 6a and 7a, and further through a clearance 12 between the upper and the lower members of the cap 2, and a clearance 13 and the like between the upper member 3 and the reserve tank body 9 o the cap 2. 
     In the oil reserve tank of a type as above-described, there is a possibility that an oil is leaked outside the rib 7 flowing through the groove 7a and further flowed to an area outside the reserve tank 1, in an event that the oil is accumulated in a space between the ribs 6 and 7 through holes 5a and 8a, and the groove 6a, and the oil reserve tank 1 is caused to incline with the presence of oil in the space. 
     In order to reliably prevent the occurrence of such oil leakage, it is required to form the labyrinth in a more complicated configuration, and thus the upper and the lower members may accordingly become increasingly complicated in their constructions. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a cap for reserve tanks which is adapted to reliably prevent the occurrence of the leakage of oil with a simplified construction. 
     The present invention provides a cap for reserve tanks, wherein upper and lower members are utilized to define a hollow portion, and an annular rib is formed in the surface of the lower member which abuts against the upper member, whereas said hollow portion is divided into inner and outer chambers by bringing the annular rib into close contact with said upper member. The inner chamber is communicated with the interior of the reserve tank body, and said inner and outer chambers are communicated with each other through a groove or a hole defined in the upper portion of said annular rib. Furthermore, said outer chamber is communicated with the exterior of said reserve tank body so as to form a labyrinth in said hollow portion, and the interior of said reserve tank body is communicated with the exterior of the reserve tank body through the labyrinth. The annular rib has its portion extended in the central direction of the annular rib, and said groove or hole is defined in the above extention of the annular rib. 
     In the cap for reserve tanks in accordance with the present invention, a groove of the rib defined in the lower member is disposed to orient upwardly, i.e., a liquid outflow port is arranged more centrally compared to the rib, which implies that said outflow port may keep its height substantially at a constant even if the reservoir is inclined, and thereby allowing the liquid to be retained in the rib. 
     As described hereinbefore, the cap for reserve tanks in accordance with the present invention has a portion of an annular rib extending towards the central direction of said rib, the annular rib forming a plurality of chambers, and a groove or a hole is provided in the extention of the annular rib so as to define a vent passage. 
     Accordingly, it is not needed for the cap for reserve tanks in accordance with the present invention to have a rather complicated labyrinth, and thereby realizing a simplified construction. Consequently, a mold can be formed inexpensively to reduce the total manufacturing cost. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 and FIG. 2 show a cap for reserve tanks in accordance with the present invention, wherein FIG. 1 is a vertical cross-sectional view of the cap and FIG. 2 is a plan view of the lower member which forms one component of the cap; 
     FIG. 3 is a vertical cross-sectional view of the oil reservoir having a traditional cap for reserve tanks. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 and FIG. 2 illustrate a cap for reserve tanks in accordance with the present invention. 
     The cap 20 or reserve tanks comprises a upper member A, a lower member 8 and a plate C. 
     The upper member A is formed from a synthetic resin such as polypropylene and the like, having a first annular rib 22 and a second annular rib 23 provided on a disk-shaped main body 21. These annular ribs 22 and 23 are formed concentrically. A groove 22a is formed on the edge of the first annular rib 22 which is arranged inwardly, and a plurality of pawls 23a is upstandably defined in the edges of the second annular rib 23 which is arranged outwardly. 
     On the other hand, the lower member B is formed from rubber, having an annular rib 25 provided on a disk-shaped main body 24. The annular rib 25 has a boss 26 formed on a part thereof which projects radially, and a groove 25a is formed in the upper surface of the boss 26. The groove 25a increases its depth progressively toward the center of the main body 24. A recess 27 is formed in the central portion of the main body 24, having an annular engaging groove 27a which is formed in the circumferential wall thereof for receiving the pawl 23a of the upper member A, and a funnelshaped hole 27b which is formed in the central portion of the recess 27. The main body 24 has an annular engaging groove 28 for receiving the circumferential portion 21a of the upper member A and an annular groove 29 for receiving the upper edge 40a of the reserve tank body 40, these annular grooves 28 and 29 being formed in the circumferential portion of the main body 24. A hole 24a is defined through the outer surface of the annular rib 25 of the main body 24. 
     A plate C is formed from a synthetic resin such as polypropropylene and the like, having an annular ribs 30a and 30b formed in the upper and the lower surfaces thereof respectively, and a hole 31 defined in the central portion thereof. 
     The upper and the lower members A and and the plate C which are thus formed are integrated as a unit in such a manner that the upper member A may mate with the lower member 8 with the plate C received in the recess 27 in the lower member, and with the pawl 23a of the upper member A received in the engaging groove 27a of the lower member 8, whereas the circumferential portion 21a of the upper member A is engaged with the engaging groove 28 of the lower member B. 
     In the cap 20 for reserve tanks which is integrated as a unit as above-described, a chamber D is defined by the plate C and the funnel-shaped hole 27 defined in the lower member 8, and a chamber E is defined in the annular rib 23 by the plate C and the upper member A, whereas a chamber F is defined by the upper member A and the lower member B between the annular ribs 23 and 25, and a chamber G is defined outwardly of the rib 25 by the upper member A and the lower member 8. 
     The cap 20 is attached to the reserve tank body 40 by receiving the upper edge 40a of the reserve tank body 40 within the groove 29 of the lower member 8. 
     In the reservoir having the cap 20 attached as above-described, a reservoir chamber 41 is opened to atmosphere via the hole 27b, the chamber D, the hole 31, the chamber E, a clearance 32, the chamber F, the groove 25a, the chamber G and the hole 24a. 
     In this reservoir, if an oil in the reservoir chamber 41 leaks to the chamber F flowing through holes 27b and 31, and the clearance 32, such oil can be immediately returned back to the reservoir chamber 41 through holes 31 and 27b. Additionally, even if the oil is leaked into the chamber F when the reservoir is in a inclined position, further outflow of the oil into the chamber G is prevented, because the inner end of the groove 25a is maintained at an elevated position. 
     Meanwhile, although the groove 25a is defined on the boss 26 so as to ensure communication between chambers F G in the above-described embodiment, the groove 25a may be naturally substituted for a hole.