I changed the fuel filter on my L3200 and bled the fuel lines via the valve a few inches above the filter. I did not loosen anything else. With a full tank the tractor starts and runs well until the fuel level drops to about 3/4 of a tank or below where the return lines enter the top of the fuel tank. Then air enters the lines and filter housing causing the tractor to sputter to a stop. Do I need to bleed the lines further or is there another cause of the problem?