Abstract:
A master cylinder having a reservoir and a cylinder formed from a single piece of molded plastic. A sight gauge is molded into a sidewall of the reservoir to permit the level of hydraulic fluid to be checked without removing the reservoir&#39;s cap. A vent hole and a replenishing hole permit hydraulic fluid to flow between the reservoir and the cylinder. A baffle is provided to deflect “geysers” that can occur during brake bleeding when hydraulic fluid is forced up through the replenishing port. In the preferred embodiment, the baffle includes an upright portion that is used to retain a float in alignment with the sight gauge.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to master cylinders used in brake systems and more specifically to a plastic master cylinder for use in hydraulic brake systems. 
     Master cylinders are components used in hydraulic brake systems. Typically they include a reservoir and a cylinder body. The reservoir portion sits above the cylinder body. The reservoir contains a supply of hydraulic fluid. Orifices connect the reservoir portion with the cylinder portion to permit the hydraulic fluid to flow between the reservoir and the cylinder. The cylinder contains a piston that moves back and forth within the cylinder to pressurize the hydraulic fluid in order to activate the brake system. The cylinder and the reservoir may be molded as a single piece of metal, or, they may be separate pieces. 
     In the past master cylinders, particularly those used in surge brake systems, have been made of cast iron or aluminum. One problem associated with such systems is that they are susceptible to corrosion. The corrosion can contaminate the brake fluid causing excessive wear of brake components. In severe cases, the corrosion may cause leakage of brake fluid, and failure of the brake system. The cylinder portion of metal master cylinders typically must be machined to meet the tolerance requirements of brake systems, which adds to the expense in manufacturing such devices. An additional drawback is the heavy weight of metal parts. Often times metal master cylinders are treated with a rust preventative, which adds to the cost of manufacture. 
     An additional problem with standard master cylinders are “geysers” that can occur during bleeding of the brake system. Residual pressure in the system can cause the brake fluid to shoot up through the ports between the cylinder and the reservoir, often shooting several inches above the master cylinder. This is messy, can damage components surrounding the master cylinder, and can even be dangerous if the brake fluid gets in the eyes of the person who removed the master cylinder cap. 
     The reservoir portion of master cylinders, especially metal ones that are formed as a single piece, are typically opaque. The level of brake fluid is checked using a dip stick, or a visual inspection of the inside of the reservoir. It would be preferable to be able to check the fluid level by a visual inspection of the outside of the master cylinder without the need to remove the cap, or use a dip stick. 
     These and other difficulties are overcome by the present invention. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, it is a primary object of the present invention to provide a master cylinder wherein the reservoir and cylinder portions of the master cylinder are molded as a single piece of plastic. 
     It is a further object of the present invention to provide a master cylinder wherein the bore portion of the cylinder is formed during the molding process without any need for secondary machining. 
     It is an additional object of the present invention to provide a master cylinder that permits the level of hydraulic fluid in the reservoir to be observed through a sight gauge molded into the reservoir, without removing the cap of the master cylinder. 
     It is another object of the present invention to provide a master cylinder that deflects geysers that can occur through the replenishing port. 
     It is yet another object of the present invention to include a baffle that deflects geysers that can occur through the replenishing port and retain a float in alignment with a sight window to permit a determination of the level of hydraulic fluid in the reservoir without removing the reservoir cap. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an isometric view of a preferred embodiment of a master cylinder according to the present invention; 
     FIG. 2 is a cut-away view of a master cylinder according to the present invention; 
     FIG. 3 is a cut-away view of a master cylinder according to the present invention including a splash baffle; 
     FIG. 4 is a rear view of a splash baffle; 
     FIG. 5 is a side view of the splash baffle shown in FIG. 4; 
     FIG. 6 is a top view of the splash baffle shown in FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Shown generally in the drawings is a master cylinder  10 . The master cylinder  10  has a reservoir  12  and a cylinder  14 . The reservoir  12  sits on top of the cylinder  14 . The cylinder  14  has a brake-line end  16  and a push-rod end  18 . A pair of holes connect the reservoir  12  with the cylinder  14 . The hole nearest to the brake-line end  16  is the replenishing port  20 , and the hole nearest to the push-rod end  18  is the vent port  22 . The replenishing port  20  is a much smaller hole than the vent port  22 . 
     The reservoir  12  has generally vertical front  23  and rear  24  sidewalls. The sidewalls  23 ,  24  may taper from bottom to top, such that they are slightly thicker where they meet the cylinder  14  than at the top of the sidewalls  23 ,  24 . This taper facilitates removing the master cylinder  10  from the mold during manufacture. Bolt sleeves  26  are provided near each corner of the reservoir  12 . The bolt sleeves  26  also preferably have sidewalls that taper from bottom to top. The rear bolt sleeves  26  are provided with retaining ridges  30  that extend towards each other parallel to the rear sidewall  24 . The rear sidewall  24  has a transparent sight gauge  28 , roughly centered on the rear sidewall  24 . Gussets  32  are provided between the bottom portion of the rear sidewall  24  and the top of the cylinder  14  to provide support for the rear sidewall  24 . Nut slots  34  are provided at the bottom of the reservoir  12  corresponding to each bolt sleeve  26 . Preferably the nut slots  34  have an interference ridge  36  that provides an interference fit with a corresponding nut to hold the nut in place in the nut slot  34 . The bottom of the reservoir  12  extends below the top of the cylinder  14 , therefore, the bottom interior of the reservoir  12  is humped in the middle, and has low spots, or troughs  38 , that run along the top of cylinder  14 . 
     The brake-line end  16  of the cylinder  14  has a threaded outlet  40  where an outlet line of the brake system may be attached. A smooth bore  42  runs from the outlet  40  towards the push-rod end  18  of the cylinder  14 . The bore  42  is preferably as straight as possible. A snap ring retaining slot  44  is formed at the push-rod end  18  of the bore  42 . The push-rod end  18  of the cylinder  14  extends beyond the front sidewall  24  of the reservoir  12 . A groove  46  is formed on the exterior of the cylinder  14  near the push-rod end  18  of the cylinder  14 . This groove  46  is for engagement with a boot that covers the push-rod assembly. 
     A splash baffle  48  may be provided. The splash baffle  48  has a baffle leg  50  that extends generally horizontally and axially from the rear sidewall  24  towards the front such that the baffle leg  50  extends over the replenishing port  20 . A generally vertical riser leg  52  joins the baffle leg  50  at the rear of the splash baffle  48 . A support gusset  54  spans between the riser leg  52  and the baffle leg  50 . The top of the riser leg  52  is provided with transversely extending wings  56 . The wings  56  engage the retaining ridges  30  on the rear bolt sleeves  26  to hold the splash baffle  48  in its operable position. Preferably, the riser leg  52  aligns with the sight gauge  28 . Spacers  63  on the wings  56  maintain the top of the baffle  48  at the proper spacing from the rear wall  24  of the reservoir  12 . A ball retaining trough  58  is formed in the riser leg  52 . A small level-indicating ball  60  is captured between the ball retaining trough  58  and the sight gauge  28 . The level-indicating ball  60  should be lighter than the hydraulic fluid, such that if floats, and should preferably be made of a highly visible material, such as a florescent orange plastic. An offset ridge  62  may be provided on the bottom of the baffle leg  50  to maintain the baffle leg  50  slightly above the top of the cylinder  14  so that it does not plug the replenishing port  20 . 
     The reservoir  12  and cylinder  14  are molded as a single unit out of long fiber glass reinforced nylon. As a result, almost all of the features of the master cylinder  10  can be formed without secondary machining. The only feature that typically requires secondary machining is the snap-ring retaining slot  44 . It is possible to form the snap-ring retaining slot in the initial molding; however it requires a retractable portion of the mold, which may not be cost effective. Importantly, the bore  42  is molded to shape without any secondary machining. 
     The sight gauge  28  is molded into the rear sidewall  24  during the molding process. Preferably the sight gauge  28  is made from a transparent plastic or glass material. The gussets  32  are necessary because the sight gauge  28  weakens the rear sidewall  24  and excessive flexing may occur if the gussets  32  are not provided. 
     The preferred use of the master cylinder  10  is in a surge brake system as might be found in a trailer. Typically, the master cylinder  10  is part of a brake actuator that bolts to the tongue of a trailer. The master cylinder  10  is bolted to the top of the actuator such that the bottom surface of the top of the actuator acts as the top cover of the reservoir  12 . Alternatively, the master cylinder  10  may be bolted directly to the hitch of the trailer such that a portion of the hitch acts as the top cover of the reservoir  12 . A gasket may be provided to seal the top of the reservoir  12 . Alternatively, a separate cover may be provided for the reservoir  12 . 
     A standard piston assembly may be used within the bore  42  of the cylinder  14 . Typically the piston assembly will include at least a front gasket  74 , a primary piston  64 , a piston cup  66 , a spring  68 , and a check valve  70 . The check valve  70  retains about 5 to 10 pounds per square inch of pressure on the hydraulic fluid, and is for use with drum brake systems to keep cup seals in the drum brakes expanded when the brakes are unactivated. The piston assembly is retained in the bore  42  by a snap ring  72  seated in the snap ring retaining slot  44 . A push rod  76  butts against the primary piston  64 . A rubber boot  78 , engaged by the boot groove  46 , covers and protects the push rod  76 . 
     When the brake assembly is in the neutral, unactivated position, the piston cup  66  is located between the replenishing port  20  and the vent port  22 . The piston cup  66  traps hydraulic fluid on the brake-line side of it, but permits hydraulic fluid to flow from the push-rod side to the brake-line side. When the brakes are activated, either by a surge of the trailer relative to the towing vehicle in the case of a surge brake, by a break-away mechanism, or by the depressing of a brake pedal in a user-controlled system, the push rod  76  pushes against the primary piston  64 , pushing the primary piston  64  and piston cup  66  against the spring  68  towards the brake-line end  16  of the cylinder  14 . As the piston cup  66  moves towards the brake-line end  16 , it covers the replenishing port  20 , and thereby pressurizes the hydraulic fluid on the brake-line side of the replenishing port  20 , thereby applying the brakes. 
     When the brakes are released, the spring  68  snaps the primary piston  64  and piston cup  66  back to the neutral position. This creates a momentary vacuum on the brake-line side of the piston cup  66 . To compensate for the vacuum, fluid flows from the reservoir  12 , through the vent port  22 , and then through holes in the primary piston  64 , and around the piston cup  66 . 
     During bleeding of the brake system, the cap to the reservoir  12  is removed. It is common in such process for the piston cup  66  to move towards the brake-line end  16  of the cylinder  14 , thereby pressurizing the hydraulic fluid on the brake-line side of the piston cup, which can cause hydraulic fluid to shoot up through the replenishing port, and possibly out of the reservoir  12 . In standard master cylinders this can be annoying and potentially even dangerous to people and harmful to surrounding parts. However, in the preferred embodiment of the present invention, the hydraulic fluid that shoots up out of the replenishing port  20  is deflected by the baffle leg  50  of the splash baffle  48 . 
     There are several other advantages to the above described system over standard existing master cylinders. Because it is made of long fiber reinforced nylon, it will not corrode, and it does not need to be painted or otherwise surface treated. Because it can be molded to size, it is not necessary to perform machining to create the bore  42  in the cylinder  12 . The molded-in sight gauge  28  permits an operator to check the level of hydraulic fluid without opening the system. This is quicker and easier, and makes contamination of the brake fluid less likely. 
     Because the mounting bolts pass all the way through the reservoir and into the nuts in the nut slots  34 , they help to provide stiffness and support to the sidewalls  24 . In standard designs, the mounting bolts thread directly into the top portion of the reservoir. 
     Troughs  38  formed on the bottom interior of the reservoir  12  provide a trap that retains any impurities that are heavier than the hydraulic fluid, and prevent them from entering the brake system.