Cooling leak

[rusty41251]

My old 1982 Honda GL500 wants to be like my R1150RT.......air cooled. It is using coolant, but does not suck any from the coolant tank. When I open the radiator cap (dead cold) there is a rush of air as there is a vacum in the system. I imagine I have cracked heads or a bad head gasket which would explain the coolant loss, but why doesn't it use any from the tank. Might the cap be bad? Or a blockage in the overflow line?

Ideas? Thanks.