[Razorback1980]

Air travels to the top of water. There are also some places inside the engine where air can get trapped. By raising the front of the car, the air bubbles travel to the radiator and on the way pass by the bleeder location. I've never used a pressure tester to bleed mine but I do put my car on a sharp incline with the front up to bleed it using the hose method outlined earlier. I've never had any problems getting all the air out when I do that.