When it comes to reworking the brakes on your car, two things seldom come into consideration: your master cylinder and pedal ratio. And those are two factors that are absolutely critical to your brake system performance. The brake pedal acts as a lever to increase the force the driver applies to the master cylinder. The master cylinder, in turn, forces fluid to the disc brake caliper pistons or drum brake wheel cylinders.
Take a close look at a brake pedal. You’ll find that the pivot point (where the pedal swivels) and the mounting point for the master cylinder pushrod are often rather different. If the length of the pedal or the distance between the pushrod mount–or a combination of both–is changed, then there is a change in the amount of (leg) force required to energize the brake master cylinder. This is the mechanical advantage of the lever or in simpler terms, the “pedal ratio.” There is a mathematical equation that helps you figure it out:
Input Force X Pedal Ratio / Brake Piston Area = PSI
The formula essentially figures out the amount of force exerted by you through your leg times the pedal ratio divided by the area of the brake piston(s). The force is measured as pounds per square inch (PSI), and we’re told that the average adult male can exert roughly 300 pounds of force (maximum) with one leg. That’s a whole bunch! Something in the order of one-third or one-half that figure is certainly more relaxed, even in a bad-to-the bone hot rod.
Let’s look at how all this applies to activating your master cylinder. A typical manual master cylinder usually requires something in the range of 600 to 1000 PSI to function properly. You have to figure out how to transform the 100 or so pounds of relaxed leg pressure (roughly one-third of the 300 PSI as we mentioned above) into pretty big PSI figures. This is accomplished by way of a lever (the brake pedal). In order to arrive at the pressure required, changing the overall length of the pedal is possible, although it’s probably a bunch easier and usually far more practical to simply shorten the distance between the pivot point and the master cylinder pushrod mount location. That’s how fab shops modify brake pedals.
But we’re not done yet.
There’s something else to think about: brake line pressure. This pressure is a totally different thing than the force you apply to the pedal. That force acts in one direction and is addressed in pounds. Pressure, on the other hand, acts in all directions and is addressed in PSI (pounds per square inch). As we pointed out above, varied (pivot length) brake pedals can be used to change the force. Inside the hydraulic system, the surface area of the piston is what is affected by pressure. Decreasing the bore size of the master cylinder increases the pressure it can build.
Pistons in master cylinders are specified by bore size, but there’s a catch. The area of a circle (or bore) is Pi–R-Squared. The area of the piston surface increases or decreases as the square of the bore size or diameter. The area of something like a common 1-1/8-inch master cylinder is approximately 0.994-inch. The area of an equally common 1.00-inch bore master cylinder is approximately 0.785-inch. Switching from the larger master cylinder to the smaller version will increase the line pressure approximately 26.5 percent assuming that pedal ratio hasn’t changed.
When the pedal force is increased or the pedal ratio is increased (or perhaps both at once), the stroke of the master cylinder is shortened (brake line pressure is unaffected). When the master cylinder piston size is increased, the output pressure of the master cylinder is decreased. A master cylinder with a smaller piston will exert more line pressure with the same amount of force (pedal ratio) than a master cylinder piston with a larger piston area. Because the brake line fluid pressure is working against the surface of the wheel cylinder (or disc brake piston), increasing the area of the cylinder will increase brake torque.
So, if your car needs some help in the stopping power department, or if there’s a need to reduce the pedal effort, you have a few options: A) decrease the master cylinder bore size; B) increase the pedal ratio; or C) increase the wheel cylinder bore size. If the pedal ratio is increased, there will be more travel at the master cylinder piston. If the master cylinder bore size is decreased, the piston has to travel further to move the same amount of fluid. Typically, a master cylinder has approximately 1-1/2-inch to 1-3/4-inch of stroke (travel). What you have to do is coordinate the pedal ratio with the bore size to arrive at approximately half of the stroke (roughly 1-inch) in order to make the brakes feel comfortable and stop the car safely.