Dual Reservoir Master Cylinder

I'm converting my 65 Barracuda to front disk brakes. I'm using an adjustable, proportional brake valve. Question (regarding the master cylinder) when routing my lines to the adjustable valve, what decides what reservoir is for the front brakes and back brakes? The new Dorman master cylinder has no front and back makings. The valve is marked front and rear in and out. I'm kind of light in this area, never gave much thought to which reservoir did what.
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