Not Just Yet, Aurthur
I wouldn't go to the extent of having the yard pull that tank with the information that "when I don't totally top off the tank it seems not to leak." Does your yard have a policy against bringing in other mechanics? If not, I'd get some recommendations and let someone else have a go at it first. This may just be either a fitting or a bad hose, either at the vent or the filler. I would sure try to trouble shoot it first. Seems to me its like having a bypass without having an EKG first.. FWIW, Rick D.