A true proportioning valve will have a different pressure output between the fronts and rears, that is the reason to use that type of valve. The line diameter will be dictated by the port/tread size at the MC, proportion valve, and wheel cylinder/caliper.
1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy
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