Soft brakes after rear disc install

Did you check / did anyone ask are you SURE the bleeders are on the top most side of the calipers, AKA switch them side for side? It' is always possible the master is not properly bled and that it is defective. Otherwise, air in the system. Try buying some inverted flare plugs and unions so you can cap off sections, starting with the rear tube where it meet the rear hose.