power brake booster mounting

[maxwedge528]

my 67 has disc/drum brake with non-boosted brakes. decided to mount a dual diaphram booster with a new master cylinder. since it set so much further away from the firewall, it would be much easier to plumb the brake lines if the proportioning valve was mounted at the firewall...question...does the proportioning valve have to be located right at the master cylinder with 2-3 inch lines between or can the valve be placed 12-15 inches away? appreciate any insight or help!