The pipes on a gravity system should not require bleeding. Replacing deoxygenated water with oxygen-bearing water promotes corrosion. There is a large tank in the attic or closet to permit the water to expand and contract with temperature changes. If it is a tank that is open to the atmosphere, make sure it is filled to the top.
If it is sealed, make sure the pressure in the system is high enough to keep water above the highest radiator and pipe. The formula is: highest pipe above the boiler divided by 2.31 equals pressure in pounds on the boiler gauge. When the water is cool, the gauge on the boiler should indicate that pressure plus 4 pounds to keep pressure in the top of the system. Use a marker to remind you of that point on the gauge.
Once you are sure the system is filled properly, you may drain some of the water at the boiler drain at the bottom of the boiler to discharge collected sediment til the water runs fairly clear. Add water to the system as you drain the water to make sure no excess air enters the system by keeping the pressure the same or a little more on that spot on the gauge.
If there is air in the radiators, there should be a small valve near the top of the radiator to let air out until some water spurts. When you are done, make sure the pressure is at the mark on the boiler gauge.
The idea is to save the water and not introduce air. If the system has not been serviced by a professional plumber experienced in heating for a long while, this is the time to have this work done by an experienced person. Have the relief valve on the boiler replaced as this saves damage and lives. Replace the boiler gauge with a new one, as this is your main indicator of proper operation. Have him check the firing controls for precise temperature. Check the combustion process for efficiency and noxious fumes to leave through a good chimney instead of into the home.