I agree with Daniel,
Hydraulic fluid is not really going to "flow" so the size of the pipes is not like exhaust or intake tubes. You want to transmit the pressure on the brake pedal to the pistons in each caliper.
The proportioning valve is just a way to restrict flow. *gasp*
It does so because you want all the tires to lock up at the same time. With weight transfer to the front tires you need more braking force on the front as compared to the rear. So if you have equal lines with no prop valve the rear tires lock up under hard braking, causing an unstable condition. Ideally you want all the tires to get to the verge of lock up simultaneously. [insert abs convo here]
All of this is simplified because we are not taking into effect other variables that influence braking. Most notably the tires and brake pads.
The factory setup is designed for the factory wheels and tires and pads. The farther from stock these parts are the faster the old ABS system becomes inadequate. Tire diameter differences front and rear cause the system to detect slip when there is none. Also with width differences, tire compound, brake pad material, etc. etc.
All this causes the factory abs to become less and less accurate in it's application. This is the reason some of the hardcore guys have been looking into/playing with the modern 4 channel setups, and retrofitting them.
If you are trying to eliminate the factory abs, then the easiest way is to get the metric m10x1.0 lines, and couplers. If not you need to use adapters between the factory metric lines and most brake parts because they run the american thread size. Sometimes it is hare to find aftermarket brake line t's in the metric thread locally. Either a junkyard or parts department for japanese cars can give you a rear brake t that you can use up front if needed. Tilton offers metric adapters that thread into their prop valve.
