Those with power brakes(front disc/rear drum)

I have a 1971 duster which originally came with 4 wheel drums. Front 9". I'm in the process of Converting the front to disc from performance online. I'm also getting their booster/master cylinder(http://www.performanceonline.com/store/customer/product.php?productid=19530&cat=291&page=1)
I will keep the rear drums for the time being. Do I have to change the stock combination valve? The kit includes risidual valves(I'm guessing if I convert to rear disc). Do I have to install them if I'm keeping rear drums?