One area of car maintenance that hot rodders, racers and custom car (and truck) builders tend to ignore is the brake master cylinder and, in particular, the actual brake pedal ratio. After all, it doesn’t make the car one bit quicker or faster, and if the thing eventually stops, why worry? Perhaps you should.
Brake pedal ratio
The critical component in the braking equation is the brake pedal ratio. In operation, the brake pedal acts as a lever to increase the force the driver applies to the master cylinder. In turn, the master cylinder forces fluid to the disc brake caliper pistons or drum brake wheel cylinders. If you examine a brake pedal, you’ll see the pivot point (where the pedal swivels) and the mounting point for the master cylinder pushrod are usually different. By varying the length of the pedal and/or the distance between the pushrod mount and the pivot, you can change how much force (from your leg) is required to energize the master cylinder. This is the “mechanical advantage” or pedal ratio. This formula will help you figure it out: Input Force x Pedal Ratio ÷ Brake Piston Area = PSI.
Mathematical babble? The arithmetic simply equates to the amount of force exerted by your leg multiplied by the pedal ratio divided by the area of the brake piston(s). FYI, the typical adult male can exert roughly 300 pounds of force (maximum) with one leg – and that’s a bunch. Something in the order of one-third or one-half that figure is obviously more comfortable, even in a hardcore race car.
The average manual (nonpower-boosted) brake master cylinder requires somewhere between 600 and 1,000 PSI to be totally effective. Somehow, 100-150 pounds of leg force has to be translated into 600-1,200 PSI. The way it’s accomplished is by way of pedal ratio. While changing the overall length of the pedal is possible, it’s often easier and far more practical to shorten the distance between the pivot point and the master cylinder pushrod mount location. That’s precisely how many race car chassis shops modify brake pedals.
Brake line pressure
Brake line pressure is a different thing than the force you apply to the pedal. Force acts in one direction and is addressed in pounds. Pressure acts in all directions against surrounding surfaces and is addressed in pounds per square inch, or PSI. “Levers” (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 hitch: The area of a circle (or bore) is πr 2. The area of the piston surface increases or decreases as the square of the bore size or diameter. For example, the area of 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.
As the pedal force or the pedal ratio (or both) is increased, the stroke of the master cylinder is shortened (brake line pressure is unaffected). When the size of the master cylinder piston increases, the output pressure of the master cylinder decreases. A smaller master cylinder piston will exert more line pressure with the same amount of force (pedal ratio) than a master cylinder piston with a larger piston area. There’s another catch: Since 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.
Improving brake performance
The bottom line is, if the stopping power of a car needs improvement, or if there’s a need to reduce the pedal effort, several options are available: (1) Decrease the master cylinder bore size; (2) Increase the pedal ratio; (3) 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 farther 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). The idea here 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, of course, to bring the car to a grinding halt.
This manual master cylinder is one of the most common you’ll find in modified applications. It is based on a Mopar configuration, and is sold by Mark Williams Enterprises and others. These master cylinders are available in at least two different bore sizes: 1.00 inch and 1 1/8-inch. (A 1 1/32-inch bore cylinder is also available from some sources. This is very close to a 1.00-inch assembly, and it’s sometimes called that.) They’re manufactured with an aluminum body along with a relatively large capacity plastic reservoir with dual outlet bores (which correctly face the driver side fender when mounted in the car).
The Mopar master cylinder has one shortcoming: the size of the outlet fittings. The front fitting is a 9/16-20-inch Inverted Flare while the rear is a 1/2-20-inch Inverted Flare. They’re not common. Lamb Components offers a solution. Lamb manufactures special adapters specifically for these master cylinders that allow an easy hook up to #3 AN fittings.
Look carefully at this piece: It’s a pushrod retainer engineered into the M-W master cylinder. The purpose? It positively retains the brake pedal pushrod. That means the pushrod can’t fall out if the pedal goes over center. And don’t laugh. It happens more regularly than you might think with modified cars.
The typical Detroit pedal assembly looks like this. This vintage Nova hanging arrangement is designed to accept the brake pedal assembly, and if equipped with a clutch, that too.
If you take a close look at this pedal, you can see two different master cylinder pushrod mount holes: one is for a booster-equipped application, while the other is for a nonboosted brake arrangement. For a late model, nonboosted manual application, many fabrication shops modify the pedal assembly by creating a mount that is higher (up the pedal) than the original. By moving the mount position higher, the pedal ratio is improved.