what angle are the rear calipers mounted at? The cossie rear caliper is designed to be mounted horizontally on top of the disc - on a S1/S2 conversion they are mounted off the top, at an angle. This means the bleed nipple drilling doesn't end up at the top of the piston cylinder, allowing air to accumulate which can't then escape.
Try bleeding the back brakes with the calipers turned horizontal (remove one of the fixing bolts and twist the caliper so it's horizontal)