[Aussie Mike]

Hydraulics is all about leverage. The difference in sizes between the size of the master cylinder bore and the caliper piston bore will give you an increase in leverage.
Here's a rough example:

You apply 100 pounds of force to your brake pedal. There is a 6:1 ratio in your pedal so that equals 600 pounds of force against the master cylinder.

The master cylinder bore is 3/4" which if you work out the piston area is 0.4418 square inches.

Pi x Radius squared: 3.14159 x (0.375x0.375) =0.4418

We want to work out the pressure in square inches so we divide 600 pounds by the area we are pushing against (0.4418 square inches) and we get 1358 Pounds per square inch. This is our line pressure.

We then feed that down a steel brake line to the caliper where it has a piston 1.5" in diameter. This has a surface area of 1.7671 square inches. We are applying 1358 pounds per square inch of pressure so we multiply that by the number of square inches of piston and that gives us 2400 pounds of clamping force.

So with these calculations we can work out how much clamping force we generate for the front and rear brakes.

When working out the clamping force you find opposed pistons cancel each other out. If you have a pair of 1.5" pistons working against each other for the calculated area you use only one piston.

So the rear has a single piston caliper and for the front example we'll assume 4 piston calipers with 1.25" pistons (2.454" square).

With a 3/4" master and our 1358 PSI of line pressure that equates to 3332 pounds of clamping force on our front brakes.

The front and the rear of the car require different amounts of braking force to stay in balance. By that I mean we want the front and rear tyres to lock up about the same time or preferably the fronts before the rears.

There are a bunch of different factors that come into play here, the main one being weight transfer. The weight of the vehicle moves to the front of the car as it decelerates forcing the front tyres onto the road and making them grip better. The front to rear weight bias of the car, and suspension setup are a factors that control weight transfer.

The diameter of the brake rotors affects how much leverage the calaper has on the disc. A larger diameter disc has more leverage than a smaller disc and requires less clamping force. The friction coeficient of the pad material comes into effect too. some pads are grippier than others. The tyres and their level of grip is a factor

So what we have with our Cobra is a car with a rearward weight bias and low center of gravity. Add that to firm suspension and you have a car that won't transfer nearly as much weight as say your average Ford Falcon. This means a Cobra can use more rear brake and less front brake in comparison with that family sedan.

What the net result of having the same size master cylinders front and rear is that we may not get the right bias between front and rear braking forces. You may end up with your balance bar wound right accross to one side. You dont have enough adjustment to get the bias right where fronts and rears are close to locking at the same time. The balance bar should really a tuning aid and you need the system balance to be pretty close in the first place.

dropping the rear master cylinder size to 5/8" gives us 0.3067 square inches. This makes 1956 PSI of line pressure for the same pedal force.

If you already have 2x 3/4" master cylinders give them a try, they may work OK. If you find you need more rear bias you can switch to the smaller rear cylinder.

A proportioning valve will only take pressure out of the rear circuit. I don't think it's legal to run one in the front circuit.

Hope this helps.

Cheers