I had a problem like this with a Nissan that used conventional brake fluid in it. The petal would fade lower after about a week. There was air in the left side. Bleed it and it would do the same thing a week later. A new master cylinder didn't help. I noticed that when I bled it the air was right there in the left wheel cylinder and came out as soon as the bleeded was cracked open. A new wheel cylinder fixed it. That's when I learned air can get in through a wheel cylinder.
Do you notice the air come out as soon as the wheel cylinder is cracked open? If so, I would think that is where it's getting in and not up top through the valve and hoses.