Possible Cause: Improper Pedal And Pushrod Geometry
With the pedal fully depressed the angle between the master cylinder and its pushrod must NEVER be less than 90 degrees (see Pushrod Geometry illustration below). If the angle falls to just 1 degree less than 90, brake pressure will decrease suddenly and dramatically. This condition is described as the pedal/pushrod assembly's being "over center."
Solution: Check And Correct The Angle
Use a small carpenter's or machinist's square to check that when the pedal is fully depressed, the effective angle between the pushrod and the pedal is never less than 90 degrees. If a visual check is not possible, measure the pressure at the caliper and look for a condition where pressure initially builds and then suddenly falls as the pedal is depressed farther. In some cases, lengthening the pushrod may alleviate this condition or it may be necessary to relocate some components to achieve the proper pedal geometry.
Special Note: When using a master cylinder that's too big (unfortunately, an all-too-common practice), it's possible that pedal travel may be reduced enough to hide an over-center condition. Then, when the master cylinder is replaced, the pedal movement is increased and an over-center problem becomes evident.
Problem: Extreme Brake Pedal Effort
The vehicle has a firm pedal, but it takes a great effort and serious leg muscles to get the car to stop.
Possible Cause 1: Improper Caliper To Master Cylinder Ratio
A simple hydraulic rule: When Volume Goes Up, Pressure Goes Down and vice versa. You must select a master cylinder that will provide the proper volume to actuate the system, while still delivering the correct pressure. A large-bore master cylinder will move more fluid but at a lower pressure. A small-bore master cylinder will create higher pressure but can't move as much fluid per stroke.
Solution 1: Optimize Piston Areas
Measure all caliper and master cylinder piston areas, re-specify master cylinder to 12-19 percent of the area of just one front caliper (see Formula A; Example 3).
Possible Cause 2: Engine Vacuum Too Low
Most vacuum boosters will produce good results only if at least 16-18 inches are available at idle.
Solution 2: Increase Vacuum
It is sometimes possible to make minor adjustments to the engine to maximize idle vacuum, but engines with even moderate performance camshafts can produce too little vacuum. In some cases it's possible to employ a master cylinder with slightly less piston area to give the system more pressure. Other alternatives include removing the vacuum booster and reconfiguring the pedal geometry for manual operation or converting to a "Hydra Boost" system (hydraulic booster driven off a pump, such as a power steering unit) or installing an electric vacuum pump, which is available from Master Power.
Possible Cause 3: Pedal Ratio Incorrect
The brake pedal ratio is determined by dividing the distance from the center of the pedal pivot to the center of the pedal pad in a straight line-see "Pedal Ratio" illustration. (Note: Most brake pedals are curved but do not calculate the curve).