I imagine there's a leak somewhere in your rear brake system -- or a LOT of air in the line. If you don't have an external leak, then I suspect you're not getting the system bled properly. Are you bleeding from the bottom up, pushing from the caliper end and catching fluid at the brake lever end? You may need to push a few syringe's worth of fluid through the system to get all the air out (if a lot of air is in the line).