I am thinking of adding the proportioning valve to the rear brakes like in the link below:

http://www.pharmacy.arizona.edu/staf...PropValve.html

I know that it is installed in the REAR brake line.

Question - when you close the valve and take pressure off of the rear brakes, does the master cyclinder add more pressure to the front brakes due to the restriction in the rear line?

Or, does the master cylinder work both lines independently?

Both lines are independent.