[Aussie Mike]

Hi Slowy,

My understanding of how a proportioning valve or a crack valve works is it's just a spring loaded piston in the circuit. The tension of the spring provides the crack point or the point at which the pressure between front and rear circuits will change.

For example if the proportioning valve has a 600 pound sq in spring pressure then the pressure in both the front and rear circuits will be the same till they reach 600PSI. At this point the line pressure overcomes the spring pressure in the proportioning valve and starts pushing the piston back. Suddenly your line pressure is split between applying pressure to the caliper and pressure against the spring loaded piston. That 600 PSI or what ever pressure it kicks in at is called the crack point or knee.

You can do the same math to work out what tension the spring would need depending on the size of the piston used just like with the master cylinders.

So what we know about springs is that most of them are fairly linear and are measured in weight per distance of compression. So for example if a spring has a rating of 600 pounds it takes 600 pounds of pressure to compress it one inch. To compress it another inch it takes another 600 pounds. That's 1200 pounds to compress it 2 inches, 1800 pounds to compress it 3 inches etc.

What this means in our brake circuit is that the pressure in the rear brakes lines won't just top out at 600 pounds. Once beyond our crack pressure it will keep increasing as the pedal is pressed harder but at a different rate to the front pressure. The linear rate of the spring comes into play. The further it compresses the greater the pressure needed.

Most proportioning valves in production cars are fixed in that knee pressure. That pressure may not be suitable for the system on a Cobra. It was designed for a family sedan with a completely different dynamic.

An adjustable proportioning valve lets you vary the amount of preload on that spring and change the crack point. You may even be able to get them with different weight springs so you can change the slope of pressure differential between front and rear circuits.

Hope this makes sense.

Cheers