wow so many ...
Some cars have LD Pumps some don't. Hyundai for example doesn't use pump for testing Evap. Cap vent valve vents (yes i get it) vents canister, not fuel tank.
Tank doesn't need to be vented because vaporised fuel creates enough pressues as it is, fuel tank is SEALED. That's why we have "Evap leak" codes if there is leak it the fuel tank. If it were your way (vent valve venting fuel tank) MIL would on constantly.
based on the early cadis, fuel vapor does not create enough pressure in the system.
there is air in the tank. i know they're sealed, but air is brought into the system.
most evap codes are caused by someone running their car while fueling up, or(in the case of gm, and nissan) bad vent or purge valves.
just had a suburban a couple of weeks ago that had a stuck open vent valve. had a taurus(the same one that i pictured that heater hose assembly for)shortly after that truck......had a bad purge line.
i know not all have LDP;s. i used to replace a lot of them on chryslers about 8 years ago. they always seemed to go bad. probably due to placement.
the point is that there is indeed air in the tank. there is no way to keep it out.