Diesel fuel expands and contracts with temperature variations. A full tank of warm fuel that cools overnight will pull a vacuum. More fuel and with a wider temperature differential will pull the most vacuum.

Likewise, warming a cold tank of fuel will create pressure. Spring & fall produce the largest swings in temperature.

I had a fuel cap on a 1994 6.5TD once that would cause the injector fuel return hoses to leak fuel about mid afternoon - after sitting in the garage all day.

Jim