So what they are telling you with these instructions is that you don't need a proportioning valve with disc/disc brakes? They make no mention of a residual pressure check valve which is in the area of 10lbs for drums and 2 lbs for discs. I would suspect that the 10lb valve is in the outlet fitting that is screwed into the combination valve. If you have a 10lb check valve on disc brakes, they will drag heavily. This modification does not make sense, and just reinforces my belief that stand alone check valves appropiate for the braking system, and an adjustable proportioning valve makes the most sense with a hot rod or custom car. One size fits all combination valves are a crap shoot if they will work properly.