Brake Pedal:  The number one cause of a very hard brake pedal is pedal ratio.  There isn't a day that goes by I solve someone's brake problem by tell him his pedal ratio is probably wrong in his street rod.  To many times a product get copied by another person or manufacturer that does not understand the engineering of the product he is copying.  I always say there are people that have good mechanical skills,  there are engineers and then there is the person thinking of the all mighty dollar.  We need a little of each in our industry, the engineer can not make the part, the welder should not engineer the part without knowing the why and what for and the copy cat should not make the part without having knowledge of the original design principle.  Everything would be fine if things were copied exactly as the original, but sometime things get modified.  I have seen a few pedals where someone have remade the upper length of the pedal shorter or the lower link longer.  This changes the pedal ratio and it has a direct effect on the output pressure of the master cylinder.  See formula for master cylinder output.

Formula for Pedal Ratio:

Pedal ratio is the ratio of leverage you brake pedal applies to the master cylinder. To determine the pedal ratio you need to measure the height of the pedal to the pivot point then divided the measurement of the pivot point to the lower arm that controls your rod to the master cylinder.

If you apply 100 pounds of pressure to the brake pedal, 100 pounds X (6 to 1) = 600 pounds of pressure.  So, if the brake pedal has been modified from its' original design the pedal ratio is effected drastically.  You can now see the pedal ratio is a "multiplier" of the pressure you apply with your foot, because this is the leverage that is applied to the master cylinder.

Now, take this same formula and substitute 2" instead of 1.5" you end up with a 4.5 to 1 ratio.  Multiply 4.5 times the 100 pounds of applied pressure and you get 450 pounds instead of 600 pound.  That half inch cost you 25 percent of your braking power.  The same thing applies when you shorten the upper measurement.

On page four is the formula to figure how much pressure your master cylinder is putting out:

Factory cars generally have two (2) pedal ratios , one for manual brakes and one for power brakes.  You will find manual pedals with ratios from 5:1 to 6.5:1 and power pedals 4:1 to 5:1.  A good example of this would be the 68 Mustang uses a longer brake pedal with a power booster.  This pedal mounts about 2.5” above the manual pedal setup.  The longer pedal reduces the pedal ratio.  If you did not change the pedal your brakes would be to sensitive, because there would be to much pressure applied to quickly.

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