The original M/C on that might have been a daul chamber or single, but being a drum/drum type M/C would have had residual valves built in. With the disc front a 10lbs residual would be a bad thing. I didn't do this particular conversion on my Falcon (a '62) but I used a '72 Maverick disc/drum M/C, which puts a residual in the rear line but not in the front. Perfect! Other than a bit of running linesit bolted in place.