If you have a pedal ratio of 5:1 with 100 pounds of force acting on a master cylinder with a 1-inch stroke, the pedal pressure is 5 × 100 = 500 pounds, while the stroke is 5 inches. Stepping up to a 6-inch stroke will give you 600 pounds of pressure, but the stroke would be longer at 6 inches (6 × 1 = 6 inches).
Mark Chichester with Master Power Brakes explained the brake pedal ratio as a mechanical lever advantage. “If the overall length of the brake pedal is 12 inches and the distance between the pivot point and where the pushrod connects is 3 inches, the brake pedal ratio is 4:1. Anything between 4:1 and 5:1 is a perfect ratio for a power brake system. A manual brake system is better suited for a pedal ratio between 5:1 and 6:1. In the example above, the distance between the pivot point and where the pushrod connects would need to be changed to 2 inches for a 6:1 ratio.”
The force multiplication of the pedal ratio can be calculated by multiplying the initial force by the ratio. For example, let’s assume the average force of 70 pounds is applied to the brake pedal. Multiply that by 4 to get the total force for a 4:1 brake pedal ratio (70 × 4 = 280 pounds of output force). Likewise, a 6:1 ratio would result in 420 pounds of output force (70 × 6 = 420). The pivot point placement and master cylinder pushrod location in higher-ratio brake pedals tend to have longer pedal travel.
Chichester also mentioned some general rules that most designers adhere to: “Whether your vehicle has power or manual brakes, pedal ratio is important. If you are experiencing a hard pedal, you should check your pedal ratio if you have converted from the vehicle’s original setup. As a general rule, your pedal ratio should not exceed 6:1 for manual brakes with a 1-inch bore master cylinder and 4:1 for power brakes with a ⅛-inch bore master cylinder.” ■
A = Distance from pi vol point to middle ut puah / pull puinl
B = Distance ham pi vol lo paini of push cm master cylinder
P = pivot point
F = Fouce or push
By performing a few quick measurements combined with a little math, the pedal ratio can be calculated, which is helpful to determine if you have enough pressure acting upon the master cylinder piston or if you have the proper amount of travel required to effectively work the master cylinder. (Photo Courtesy Wilwood Engineering Inc.)
The fluid used in a brake system undergoes tremendous pressure and heat cycles. Like engine oils, there are several different grades of brake fluid to choose from to match your goals and driving conditions. The most common fluids are specified by the Department of Transportation: DOT3, DOT4, and DOT5.
DOT3 is the base fluid and is not as capable of performing in higher-performance applications compared to the DOT4 blend.
DOT4 brake fluid is considered a higher-performance brake fluid because of the addition of borate esters, which improve the dry and wet points of the fluid. While DOT4 brake fluids are more stable and have a higher boiling point initially, once the fluid begins to absorb water, its boiling point will fall off more rapidly than most DOT3 brake fluids.
Use care so the brake fluid is not exposed to open atmosphere where it can pull water molecules from the air. DOT4 brake fluids must have a minimum dry boiling point of 446°F (230°C) and a minimum wet boiling point of 311°F (155°C) by US Federal Motor Vehicle Safety Standards (FMVSS).
The dry boiling point of brake fluid is described as the boiling temperature of brake fluid from an unopened container. The wet boiling point refers to the brake fluid boiling point after it has absorbed 3.7 percent water by volume. Most experts estimate that brake fluids reach this point just past the two-year mark. This is why the experts recommend changing brake fluid every two years.
DOT5.1 brake fluids also have a blend of ethyl glycol and borate ester, but this blend meets the standards of the silicone-based DOT5 brake fluid. In simple terms, the DOT5.1 brake fluid is a DOT4 fluid that meets the DOT5 standards. Because the blend is essentially the same, DOT5.1 and DOT3 and DOT4 brake fluids are compatible.
The Department of Transportation rates brake fluid in several classifications; always check with a brake manufacturer for its recommendation. DOT3 provides the characteristics for most cars; for spirited performance and on-track days, DOT4 is recommended.
DOT5 is a silicone-based fluid which, while not designed for high-performance driving, is a favorite among hot rodders because it will not remove paint if it gets spilled or splashed compared to DOT3 and 4, which are highly corrosive to paints and coatings.
Brake fluid must hold its operating parameters through quite a few different requirements. It must have an extremely low freezing point as well as an extreme boiling point. During those varied operating parameters, it must have a constant viscosity and not maintain its compressibility. Add to the list its ability to lubricate the moving components and prevent corrosion, it makes for a pretty tall order. Believe it or not, the fluid also needs to be able to absorb any moisture that collects in the system.
It’s always a best practice to fill a fresh brake system with new brake fluid. A brake fluid canister that has been open on a shelf will absorb moisture in the air. By pouring it into an existing brake system, you would be introducing more moisture. It is important to note that mixing DOT3 and DOT4 fluids is acceptable. However, never mix a DOT5 synthetic fluid with a glycol-based fluid. Component deterioration as well as the transfer of pressure will not function as well when combined.
TBM Brakes offers a high-end brake fluid for the most severe applications. This DOT5.1 fluid is suitable for long races and whenever critical conditions are present. Rated at a 612°F dry boiling point, it has slow moisture absorption and is compatible with all DOT3 and 4 brake fluids.
Master Cylinder
The master cylinder is one of the most important components of your brake system and plays a direct role in the resulting pedal effort, modulation, and the overall braking effectiveness of the system. When selecting a master cylinder, it is highly advised to use the recommendation of the brake system manufacturer due to the number of variables in caliper, booster, or drum designs and fitments.
The master cylinder of the braking system is the heart of the system. It has a reservoir to hold the brake fluid and it converts the mechanical effort from the brake pedal into hydraulic pressure to activate the brake calipers or drums. To simplify its operation, inside the cylinder is a piston that is pushed through a bore by a pushrod connected to the brake pedal assembly. As the piston is pushed into the cylinder, it pressurizes the system with brake fluid.
Within the cylinder, there are small ports that direct the fluid to the proper brake circuit. This creates pressure that acts upon a slave cylinder (wheel cylinder or caliper piston), which in turn pushes the brake pad against the rotating drum or brake rotor.
Many older vehicles were equipped with a single-channel master cylinder, which should be one of the first items updated on any vehicle that will be driven. The reason is simple: safety. A single reservoir is responsible for maintaining both the front and rear brake circuits, and if one circuit is compromised, it will affect the operation of the other. For example, if a rear brake line fails, not only would the rear brakes be inoperative, the front brakes would also diminish! A single reservoir can easily be upgraded to a dual master cylinder.
Upgrading to a dual reservoir in place of a single-chamber unit is an important upgrade. This example does not have any power assist and simply required a new line and proportioning valve to slow the application of the rear brakes. The bracket shown in the