Dugley,
I have literally spent
years working on that exact problem with my Cobra. I have also very nearly killed myself testing the brakes. I have a bunch of Wilwood master cylinders and lot of other parts that didn't work out in a cabinet. Some day when I retire, I'll try to sell them on eBay.
The master cylinder is not your problem. It is the brake pads. Some pads grip the rotors more aggressively than others. New pads almost never grip and take a few hundred miles to wear in and start working well. A lot of them will work for a while, then lose their grip as they age. I have tried lots of brake pads. I have found that Ferodo pads will fit my Jaguar rear calipers with Kurt Hamilton's vented rotors and lock up the rears pretty well. I cannot find Ferodo pads to fit my front calipers. I have tried many other brands and am currently using Porterfield's. These grip reliabily and never fade.
The simple fact is, that without power assist on the front brakes, it takes a LOT of leg to stop a Cobra. I could not find the right combination to enable my leg lock up all four wheels. I have Wilwood balance bar and two master cylinders with an adjustment knob so that I can adjust the bias from front to rear. I
can vividly describe how quickly a Cobra will spin around if your rear brakes lock up before the fronts. It is very scary and could easily kill you. This is to be avoided at all costs.
I have not been able to find the right combination of components to lock up my Cobra's wide sticky front tires (Mickey Thompson 255/60/15's) without power assist. The front tires of a car do 80 to 90 percent of the work to stop the car. Many Cobra's and hot rods are been fitted with some form of power assist for this reason. I use a vacuum powered remote brake booster mounted in front of my engine to do the job. Like the one in this article:
http://www.race-dezert.com/forum/sho...e-Booster-Pics. It is not pretty, but it works really well. However, that's not the whole story. With my new engine's hydraulic roller cam, I have a lot of vacuum and too much front brake bias. So, I also had to install a Wilwood adjustable proportioning valve. With that, I can set the booster's maximum pressure so that I can balance the brakes. There are also hydraulic brake boosters that use power from a car's power steering pump to help apply pressure, but I don't have power steering. Someday, when I have an extra $1,200, I will buy an electric brake booster like this:
http://www.speedwaymotors.com/Electr...Disc,8335.html.
To adjust the brake bias, I waited for the right day when we had a good rain shower (that stopped) to take my car to a big parking lot. On wet pavement I don't flat spot the tires. I get the car up to about 30 and turn the wheel into a gentle turn then hit the brakes really hard. If the car spins around, you have too much rear bias. If it straightens out, you have too much front bias. If it slides at an angle you have it just right. It has to do that in both directions.
The final chapter is about the brake bias adjuster. If I'm on a rally, the gas tank is full and my Co driver’s 240 pound butt is in the car, it goes all the way to the rear and the rear's still do not lock up. If I am autocrossing alone with a light fuel load, I move to it to the center and keep working it between runs to get it exactly right. I hope this helps you.
RS