I mocked up a prototype manual brake system on a 1991 Firebird. This system requires a 2.0 hole to be drilled in the firewall and a 31/64 hole to be drilled in the pedal.
The 2.0 hole drilled in the firewall allows the use of the upper two holes that the brake booster bracket utilized and allows a master cylinder to bolt up.
This arrangement allows for a 6 to 1 pedal ratio. With a true 6 to 1 pedal ratio and using the upper bolt holes in the firewall, you also get good pushrod alignment with the master cylinder piston.
The 31/64 hole, that will be drilled into the brake pedal arm, is good for the ½ stud/pin to be pressed into the pedal with an optimal interference fit with zero stud/pin play.
To utilize the ½ stud/pin that is pressed into the brake pedal arm, I made a custom clevis with a ½ hole for the stud/pin. The clevis is machined to use a 5/16 diameter pushrod with fine threads.
Here are the items used for the pushrod assembly.
A custom CNC aluminum plate was machined to cover up the original power booster hole and allow GM, Ford, or Mopar style master cylinder to be utilized in the upper holes. A retention cup will also be fabricated and pressed into the plate to retain the pushrod so it doesn't fall out the back of the master cylinder.
Here is the plate bolted to the firewall.
To utilize a Ford or Mopar style master cylinder, the upper passenger side mounting bolt hole in the firewall will have to drilled out to a ½ in diameter (before the adapter plate is installed) because these master cylinders have a narrower bolt pattern than a GM master cylinder.
Because the third generation F-bodies have significant firewall flex, the prototype manual brake setup utilizes a ½ thick aluminum plate under the dash between the firewall and the brake pedal assembly. This plate overlaps the contours of the firewall to reduce firewall flex significantly.
I will utilize a 7/8 bore master cylinder because the smaller diameter bore allows for greater pressure in the system and is sized to provide great clamping forces for the stock 2.5 piston calipers.
Here is the system mocked up on the firewall utilizing an old 7/8 bore Mopar style master cylinder I had lying around.
From the factory, the stock calipers are LOW drag. LOW drag calipers will require a step bore master cylinder. Step bore master cylinders have two bores. The secondary, pressure bore is 24mm, which in my opinion, is too large to operate manual brakes. Also, the master cylinder does not have a mechanism to retain the pushrod. If the pushrod falls away from the master cylinder piston, the car will loose all its brakes.
NON low drag (normal) calipers are available that will bolt in to systems utilizing the stock 10.5" front brakes. I like the Centric brand rebuilt units.
The part numbers are 14162066 and 14162065 (left and right hand side) and have the stock 2.5 piston diameters. AFCO just released some brand new cast iron castings with stock 2.5 bore. The part numbers for these are 6635003 and 6635004 for left and right hand side calipers.
The ports on the Mopar style master cylinder are 3/8-24 and will utilize this size line to mate up to the stock prop valve. I just need to figure out the prop valve inlet thread size to see if there are any adapters to mate to the 3/8-24 line fittings.