The larger bore diameter of the '99+ master cylinder corresponds to a 13% increase in the cross-sectional area of the bore, which translates to an 11% reduction in line pressure for a given applied force on the brake pedal and a 13% increase in volumetric flow of brake fluid for a given amount of pedal travel. The lower line pressure enables a finer resolution of braking force modulation (i.e., the resolution of applied force at the pedal relative to the clamping force generated at the caliper is increased). The larger volumetric flow results in reduced pedal travel and firmer pedal feel.
The result of this upgrade is far less pedal travel, a pedal that is much firmer (not too firm, though) and finer modulation resolution. The pedal travel is reduced considerably, placing the pedal slightly above the at-rest position of the gas pedal. Total pedal travel is reduced by about 50% over the '96-'98 master cylinder. Because the pedal is much firmer an higher, pedal travel associated with modulation of braking force takes place over a much narrower range of pedal travel and completely above the at-rest position of the gas pedal. With the 96-'98 master cylinder, any productive braking/modulation was below the position of the gas pedal even after 'having lowered the at rest position of the gas pedal for pedal position and foot transition between pedals. This made heel-toe driving and left foot braking less than optimal/desireable.
As shown below, there are a few differences between the '96-'98 Hydro-Boost master cylinder and the 99+ Hydro-Boost master cylinder. These differences necessitate slight adaptation/modification being required to facilitate installation of the 99+ master cylinder on a 96-98 GT or Cobra.
On master cylinders for '96-'98 GT's and '96-'98 Cobras, there is a 'yet to be confirmed' difference (although, confirmed that it is NOT bore diameter) between the GT and Cobra master cylinders. Some aftermarket parts catalogs list the same part number for '96-'98 GT and Cobras and some list different numbers for the two car models. One explanation was that there is an internal design difference that affects the volume ratio of the front-to-rear circuits and that Ford uses the different size ports to physically distinquish between master cylinders for '96-'98 GT's and '96-'98 Cobras.
All '99+ Mustangs with V-8's and traction control use the same master cylinder, which is different than the master cylinder used on '99+ Mustangs without non-traction control/ABS. It has not been confirmed whether the master cylinder used on traction control/ABS cars have the same size or different size front and rear ports than do '99+ Mustangs without non-traction control/ABS. Thus, it has not been confirmed whether ALL master cylinders for '99+ Mustangs with V-8 engines have different size front and rear ports.
It should be noted that '96-'03 Mustangs with V-8 engines use a Hydro-Boost assisted brake system, whereas '96-'03 Mustangs with V-6 engines use a vacuum-assist brake system. The master cylinders used in the Hydro-Boost brake systems and master cylinders used in vacuum assist brake systems are not interchangable.
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