Technical - Figuring Master Cylinders & Pedal Ratio

The average manual (non-power boosted) master cylinder will
require somewhere between 600 to 1,000 PSI to be totally
effective. Somehow you have to translate the 100-150
pounds of leg force 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 usually
easier (and far more
practical) to shorten
the distance between
the pivot point
and the master
cylinder pushrod
mount location.
That's precisely
how MPR modifies
late model
brake pedals in
their power-to-manual
brake conversions.

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. In contrast, most late model cars are designed with power brakes and that can spell trouble when it comes to pedal ratio.

So far, so good. 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 as outlined above) 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 will increase 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 piàr-squared. The area of the piston surface increases or decreases with the square of the bore size or diameter. The area of a 1-1/8-inch master cylinder is approximately 0.994-inch. The area of a 1.00-inch bore master cylinder is approximately 0.785-inch. As a result, 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.

According to Mark Williams, the normal bore size in a drag racecar where rear two or four piston (1-3/4") brakes are used on something like a dragster (no front brakes), is a single 7/8-inch master cylinder assembly. If a single piston front design brake is used in conjunction with a pair of four piston rear calipers, a 1.00-inch tandem master cylinder will

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work. In applications where four piston brakes are used front and rear, a 1-1/8-inch bore tandem master cylinder will work. Mark points out an important factor with regard to master cylinders. "Attachment to frame rails or the roll bar is preferred to ensure a solid mount. Often times, a spongyÇ brake can be attributed to master cylinder deflection. Do not use the firewall sheet metal as the sole mount". This is an interesting point, especially in light of old racing articles regarding the structural integrity of firewalls (the "oil can syndrome"). Many experienced racers and chassis builders have found the cause of cracking in the area of the master cylinder had absolutely nothing to do with the structural integrity of the roll cage or the chassis. Instead, it is the force of the driver's leg against the brake pedal that creates the cracking. Good examples of this include many vintage cars with manual (non-boosted) brakes that have never been modified. The sheet metal surrounding the master cylinder eventually fatigues in these cars, even though they were never modified (meanwhile, similar un-modified cars with power brakes seldom have evidence of firewall fatigue). That's why sharp door car chassis builders routinely brace the master cylinder to the roll cage (or the forward engine compartment hoop).