[Aussie Mike]

A lot of the stuff I've read about getting Cobra brakes working right is band-aid fixes and I find that a little disturbing.

I would strongly advise against blocking off one port of the master cylinder and converting the system to single circuit. Apart from being illegal under the ADR's the whole reason for having dual brake circuits is safety. If you pop a seal on a caliper or burst a line on a single circuit system you have no brakes at all. At least with a dual circuit system you will still have front brakes or rear brakes in the event of a failure.

The problem with many Cobra brake systems is badly matched parts. Brakes are simple friction and hydraulics. In it's simpelest form you are turning mechanical effort from your foot into clamping force on the discs to create friction and slow the car. If you add a brake booster to the system you are effectively adding extra mechanical force at the pedal. If this is making the brakes touchy without the feel at the pedal then you probably have too much mechanical advantage in your system.

There are some typical figures I have at home of what a good pedal pressure should be. I think it was around 50KG but I'll check and get back to you.

So you take this 50KG your foot applies to the pedal and multiply it by the pedal ratio (probably around 5 or 6:1 for a boosted system). This gives you 250-300KG of force on the brake booster push rod. The brake booster has 1 or 2 diaphrams inside that are pulled in by engine vacume. There's a one way check valve to hold the vacume in the booster so you still have boost when your engine has low manifold vacume ie accelerating or stalled. The surface area of the booster diaphram or diaphrams can be quite large so it can add quite a bit of extra force on the master cylinder when you press that pedal.

I believe the force the booster exerts is also modulated by the amount of pressure applied on it by the pedal. This could be a fault with yours.

Your master cylinder is a dual circuit design meaning that inside it there are 2 pistons, one in front of the other. front to rear brake bias is often controled in these systems by either having different bore sizes i.e. the front piston will be smaller than the rear and feed the rear brakes with their smaller pistons. Some have a proportioning valve inside them that uses a heavy spring between the pistons to absorb some of that pedal pressure and give the desired pressure difference the calipers need from front to rear.

The problem with a lot of our cars is that they are built from donor parts. A MC from one car and calipers from another all bolted into a car that weighs much less and has a different front rear weight bias. It's highly unlikely that all these parts will suit each other as the components did in their parent cars.

So if your brakes a locking with only light pressure either your pedal ratio or the booster are giving you too much mechanical leverage. You shold perhaps look at removing some of the mechanical advantage by changing your pedal ratio or booster size. Alternatively you could change your hydraulic leverage. A smaller MC bore will give more ratio between the MC and the caliper pistons. A larger bore MC will give a smaller ratio and less pressure at the caliper.

Brake proportion is the other problem that gets people in trouble. The back brakes lock up before the fronts or the front brakes are doing all the work and the rears do nothing. Ideally you want the fronts to lock shortly before the rears. This will keep the car tracking straight under hard braking. I've heard of some racing drivers that set their brakes to lock the rears before the front so they can back the car into a corner and get it to turn in but that's not for road cars or us mere mortals.

Done right the brake proportion needs to be designed into the system either by using the right mix of piston sizes front and rear or a stepped MC (or seperate MC for front and rear circuits). The proportioning valve should be an aid to fine tuning the system but the system needs to be in the ball park in the first place.

It can be an expensive proposition buying a set of calipers with right size pistons however there are some reasonably priced adjustable proportioning valves from Wilwood and Tilton. One can be incorporated into the rear or front line to drop some pressure to those calipers. These are basically a spring loaded piston that absorbs some of that line pressure. Varying the preload on the spring varies how much pressure they absorb. Wilwood quote between 100 and 1000 PSI for theirs.

Get your pedal pressure issue sorted first and then you can chase your brake proportion when you can feel what's going on.

Cheers

*edited for spelling