[Quicksilver]

Vacuum bleeding can pull air past seals. The seals are designed to contain pressure not vacuum.

That said, unless there is a huge quantity of air in the system it won't change the bias unless the pistons in the master cylinder are hitting each other. The master cylinder produces the same pressure out of both ports. Even if the quantity of fluid and/or air in the front and rear systems is different they will both have the same pressure.

Either there is a change in the friction level in the front and rear brakes, there is a change in the effort required to actuate the calipers in one end. Or there is a huge quantity of air in the systems so the primary piston is hitting the secondary piston (which would require large pedal movement).