^^What he is trying to say is, the original distro block is NOT a proportioning valve. You either need to add an aftermarket one, or find / buy one like the original 73/ later stuff. Adjustable is attractive as it allows, well, "adjustment." This could be due to tire differences, brake mismatch front to rear, or even lining type.
"Why can't" you fix the leak in the one you have? Or replace it with another?