Air could be coming in from caliper seal also. This happened to me on an older car where I applied brake pressure it made seal expand and then sucked in air when releasing brakes due to seal moving with piston back into bore. Replaced calipers on that occasion and fixed issue.
When you bleed brakes take notice to which area is full of air. You may only be getting air from left caliper, only right, or only MC. That will help narrow down where your issue is coming from. At this point you just need to start working on it.
You can also try the syringe method and push fluid up through lines from calipers. This will help push all air to the top. Many ways to skin a cat here..