When plumbing hard brake lines...
When plumbing hard brake lines there are only two acceptable types of tubing: steel and stainless steel. Flex lines should be DOT-approved rubber hoses or braided lines.
Residual valves maintain a small amount of pressure in drum brake systems to keep the wheel cylinder cups expanded. This prevents air from being drawn into the system and allows the brakes to react quicker. A 10-pound valve is common in drum brake systems.
Normally, disc brake systems don't have residual pressure valves, however when the master cylinder is mounted below the floor, and is lower than the calipers, a 2-pound valve is used to prevent the calipers from draining fluid back to the master cylinder.
A combination valve incorporates metering and proportioning functions into one valve. These are available for disc/drum or drum/drum systems and often have warning light provisions to indicate if one half of a dual brake system has lost pressure.
Adjustable Proportioning Valves
Adjustable proportioning valves are often found on race cars to adjust brake bias as fuel load or track conditions change. On trucks they are often used to "tune" front-to-rear brake balance.
The first thing that needs to be understood is that the size of the brake lines has no effect on system pressure. With that said, only steel or stainless steel lines should be used and braided line should only be used between the chassis and wheels. Plumbing an entire chassis with braided line can cause a spongy pedal because it will swell slightly in use
When routing brake lines keep them away from heat sources, points of abrasion, and avoid large vertical loops, like over the rear axle, which can cause the system to trap air and make it difficult to bleed.
Brake Buying Tips
First and foremost realize that the major players in the brake business have taken a systems approach to their kits. That means that mixing and matching components from a variety of suppliers may not provide the desired results and you could wind up with a system that is less than the sum of its parts by trying to outguess the manufacturers.
Finally, talk to suppliers about your specific needs and your budget, chances are they've dealt with the same requirements many times before and will have some recommendations.