Ever leave one open slightly and watch it for a while as the motor runs, fully warmed up?
Does it keep blowing bubbles or solid liquid stream? Never-ending air while running would be indicative of a coolant jacket leak.
I've found you can't get a good bleed with a cold motor...I fill, bleed, run to operating temp, shut off, wait ten minutes, bleed while filling, restart, run to operating temp, repeat, etc, until there's no more air. Need the heater on, so air doesn't hide there, too.
Never had to do anything to the motor as far as positioning...