filling the system

My very old three-family home has three furnaces, two water, one steam. The semi-finished attic is on the same water furnace as the basement apartment. The basement heats great; the attic does not heat at all. Two different circulators, two different circuits.



I'm pretty sure the problem is that there is no water in the pipes in the attic. Playing around with the valves, I can't drain very much water from the attic pipes. In fact, it seems that it only fills to the same height as the basement -- the basement pipes go up to the basement ceiling, and that's about how much water comes out of the attic circuit.



The water refills automatically -- but when I close the valve below the attic drain, and then drain that attic pipe, the water doesn't run until I reopen the valve -- from which I deduce that there is not a circuit of water: whatever tells my water to stop filling sees that the out going to the attic is still at its appropriate height -- basement ceiling height -- even though I'm draining the return, and it does not fill further than that.



The attic has baseboard radiators. I don't see any way to fill up there.



How do I get these pipes to fill?



Thanks.