My circulator started leaking whin I fired up my heat for the first time this year. I replaced the water circulator refilled the system to 20 lbs the result, half of my baseboards do not heat. All are on the second floor. It is a one zone system. Only 960 sq ft. I have three return lines for the water two never get hot. I have blead the units many times. There are no valves anywhere, no special devices and I can see most pipes in the basement. I can not make the second floor baseboards get hot. I can feel the heat leaving the boiler from the one main pipe in the basement. One hot water out three returns only one gets hot water drains from all base boards.