A 10# residual valve should be run on the rear drum setup to restrict the brake springs from pulling the shoes too far back away from the drum. The residual valve maintains enough pressure in the rear circuit to accomplish this.
Without them, you would have a difficult time getting and maintaining a good firm pedal without a lot of pedal travel.
Once again, there are some really great sites that take the mystery out of brake conversions:
http://www.ecihotrodbrakes.com http://www.mpbrakes.com http://classicperform.com Hope this helps.
