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Wayne your problem is either a crack or one of the gaskets which are under either the fuel senders or inspection covers... if in fact it is leaking only when you fill them all the way up... then I would suspect that it would be one of the gas guage sender covers... their I would look for staining around when full... and you should see it by removing the cover... 4 screws... and looking at them when the tanks are full... they are kinda weeping leaks...as the gaskets get dry and old... so they won't be streamers...
It is also dye in the gas not anti corrosion stuff... smile... good luck GMAs...
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