Proportioning Valve - which one?

What size drums on the back?,and What size tires ?
If your drums are 9 and tires are 225 frt/275 rear, you won't even need a valve.
If your drums are 10 and tires are 235frt/295 rear, you won't need a valve.
if your drums are 10 and rear tires are 275s, you will probably not need a valve.
The factory valve was set up conservatively for 4 same size tires.
The point is; try it without and see where you are at.
I run line 3