I have a firewall mounted MC with 4 wheel disc brakes. I have two inline 2lb residual valves that are inline and close to the MC. You need two 2 lb for 4 wheel discs. And my adjustable proportioning valve is on the side of the frame near the front of the rear tire.
You can get away with not running the residual valves, but when you hit the pedal the first time it will be soft. Then when youhit it agian it will get alittle harder. The valves keeps fluid from draining back.