OK.
First, Ed is correct.
The proportioning valve should be for the rear brakes. Can be in the rear line wherever you wish to install it, locationwise.
Reason, again, as Ed says, you definately don't want the rear brakes locking up before the fronts. You will swap ends before you can think about it.
Those of you that have the valve open (off), can possibly be for a couple reasons. One, if foot braking, you won't have enough pressure to hold the rear wheels to leave on the converter. Also, this can be by having different (smaller) rear brakes on the rear.
In all the cars I have and do build, I install the valve. I'd rather see someone have to open it up some than not have it and lock the rear brakes.
Now, I have been a Strange dealer for longer that I'd want to admit, so if there is some misunderstanding of info re installation, I' d like you to send me either a PM or e-mail me at
info@smithperformance.com and I'll follow up.
hope this helps.
FJ