I just installed new rotors and calipers at all four corners. I cannot get the rear brakes to bleed. I bench bled the new master cylinder with no issues. The old rears were not functioning as the pads had rusted to the calipers, but there was brake fluid in both calipers and lines. What should I check? Should I just have a shop use their vacuum bleeder? I was able to get the fronts bled so they work.