If you clamp the rear and get a solid pedal means you have air in the rear. A dual resoiviour is a safety item. Now would be a good time to upgrade the system. Did you test the front? The theory of adjusting the brakes to the max is if you adjust the brakes tight so the piston can't move then fluid and air might be forced out. If the issue is just in the rear and you don't want to upgrade the system, then you might clamp off the frt then bleed the rear. Doing this will force more fluid towards the rear in hopes to getting the air out.