Mine was more noticeable at half or less
Wish I could answer that one with reasonable understanding but mine too emitted more fumes at half tank or less but leaked none the less at either level. If in fact you had a venting problem, lack of let's say, you'd either starve the engine of fuel as well as pull a vac on the fuel tank. Over venting would cause fuel to find a way out as a liquid during a full tank.
Check the filter and pump connections first, and let us know what you find. Unfortunately the engine does need to be running to see the seepage coming from either, so please be careful.
"Tell me and I will listen, Teach me and I will learn, Show me and I will accomplish, Involve me and I will succeed."
'84 300SD 256,000 Gold on Brown (Mileage Award)
'86 300E 246,000 Blue on Tan