Running on prime w/ the vac line capped at both ends will by-pass the petc0ck vac,
but if you run o.k. on reserve, it operates on vac just like the main, so that means the petc0ck gas screen in the gas tank is blocked up, or the ball behind the petc0ck selector knob is plugged. Not a likely thing, as you will usually see it plug up the reserve area and not the main, due to lack of use, unless the PO always ran it on reserve.
You might want to just pull it, take it apart, and see whats going on in there. Not to bad a job, just make sure to drain the gas tank first...
Hope that's not too confusing...
Here's a link to the parts fiche, if that's any help.
http://fiche.ronayers.com/Index.cfm/Module/Main/TypeID/26/Type/Motorcycle/Mak...