Weeds: The residual valves keeps constant fluid pressure to your brakes instead of the fluid being returned to the master cylinder. Discs use 2 lb. valves and drums use 10 lb. valves (each in the line between the master cylinder and the brake). This is if you decide to do the chassis mount master cylinder where your M.C. is level with or below your brake lines.

On the topic of proportioning valves, I reused an old proportioning valve on a Mustang set up I did and it almost cost me the car because the fluid wouldn't hold on hills (thank god for e-brakes!) I swapped it out for the one summit has (p. 212 in the new mag if you get it). It runs about 40 bucks and was worth it. Essentially is allows you to keep full pressure on your front brakes and allows you to control the pressure to your rear brakes.

Even if you decide not to do the chassis mount M.C., you should still replace the proportioning valve in my opinion. It will save you some headaches in the future!

Hope this helps! Best of luck!

Rumble Old Truck, Rumble!