Yes bleeding needs to be done each season. Make sure the pressure in the boiler is at least 12 psi for a 2 story. If you have a 2 story with a basement figure at least 15 psi. After you bleed at least 2 radiators, recheck the pressure and refill if necessary. There should be a pressure gauge on the boiler which indicates temp and pressure. Use a cup and bleed the air into the cup, so if it spitts water and air it will not go all over the place. Each radiator has a bleed valve on the highest part of the radiator.