Yeah definatly between 3 & 4mm difference, my 1st set up like you are talking about, had a proportioning valve which differs from a bias valve from the little info I picked up, in that its set up out of the car, and not incar adjustable........from what I gathered it was to ensure braking at the rear in terms of left to right handside was ballanced!

I now run both a bias controll incar(front to rear), and proportioning valve, there was a lot of work in it Yeah! but the handbrake was plumbed into the system at the same time, and new piping all round!

With it being something as important as braking, do get it done professionaly, as so much can go wrong! I've hear of far to many horror storys with either over braked rear's or unbalanced and over braked rears!

Please see the above as merely guidance and not fact, but am 99% confident what Im typing is correct!

Best of luck!