brake bias

[Nitro]

My 65 Malibu SS locks up the rears before the fronts. I've replaced everything inside the drums, except the backing plate. The wear pads on the backing plate have been dressed and lubed. The drums have been turned. I have replaced the single piston master cylinder, and single diaphram booster.
Everything is as it rolled of the line, I believe.
The 65 didn't have a proportioning valve. Proportioning was accomplished with larger wheel cylinders and wider front brake shoes.
The master cylinder has a single line that goes to a distribution block, with two line feeding the front brake, and a third line that goes to the rears. I know I could put a prop valve in the rear line, but it should work the way it came from Chevy.
The last time I replaced a rear brake line was 1969. I seem to remember finding a small piece that slid inside the double flare, that reduced the rear line way down. It was a long time ago!
Thanks, Nitro