I think most of the places an air leak could occur are in the remainder of the carburetor rather than the primer base, but since it happened when somebody made an amateur repair of the primer, I'd begin by inspecting that. However I wouldn't replace it without testing it in detail. Here is the Zama workshop manual that covers your carburetor:
http://www.zamacarb.com/pdfs/TechGuide_2007.pdf

Here is the primer pump:
[Linked Image]

As you can see, if the outlet check valve (C) is stuck open, pushing and releasing the bulb will just push air out through the return hose, then suck the same air back in. This is my favourite explanation in this case because the pump was worked on immediately before the problem arose. However, a largish air leak into either of the carburetor chambers would also produce more or less what is happening: air would be sucked into the pump through its inlet check valve (B) and pumped into the fuel tank through the fuel return line.

Why not just remove the bulb, then try to blow into the primer pump through the fuel return line? If you can, valve (C) is stuck open. If you can't, it's not.