Any time you go to bleed the rear brakes and you get at little fluid, generally means you have a lot
of air in the system. You have to keep bleeding until you starting building up pressure. If you cannot get a build up of pressure means there is a opening so where in the system. You need to check all connection, wheel cylinders that a seals have not pop out and brakes adjusted properly. If you decided to use synthetic brake fluid that can created leaks. I would no recommend using synthetic brake fluid.\