using drum MC on front disc setup

Easy. The later (dual) drum units normally have what is called a residual pressure valve built into each line fitting. This is the brass seat you see when you remove the line. You can find a small sheet metal screw, thread in there, and then pull out with a claw hammer. Remove everything behind it, then reinstall the brass seat. Leave the one in for drums. Normally, the rear line goes to front brakes, front line goes to rear (drum) brakes