Correct me if I'm wrong but dual circuit brakes should have two brake lines going from the master cylinder, one goes to the c\back wheels the other to the front.
Single circuit has only one line leaving the master cylinder it goes into a T piece on the firewall and one line goes to the front brakes the other to the back. Because the front brakes should activate more than the rear brakes there is also a brake proportioning valve on the rear subframe this reduces the amount of brake pressure applied to the rear brakes (more brake pressure goes to the front)
Hope this helps
