it means your vent is blocked , lines blocked with rust , kinked etc
as to dons comment about cannisters i dont think he has a clue as to how they operate and what their function is ,
there is no downside to running one you need a vent to allow air into tank so fuel can come out , no air in and tank becomes a vacuum
if you shake a tank of gas it quickly pressurizes , so you need to vent the pressure
so now you know why you need a vent but venting to atmosphere is bad as we've all been told and it smells up a garage, the vent is at the top of then sender unit and its possible and likely some fuel will get into the lines not just fuel vapor and if tank is filled and parked off level theres also possibility to push raw fuel up the vent hose
the canister is attached to the vent and its full of charcoal which traps the fumes and stores it in the charcoal like a sponge , theres a solenoid on the canister connected to engine with a vac hose only activates non decell and it opens and allows the engine to draw from the cannister allowing the stored fuel to be burned out as vapors and refresh the charcoal
fyi canister isnt a sealed can , theres a round flat cottony filter on the underside that needs replacing now and then , if it clogs the cannister doesnt flow freely to allow vaporization to refresh the charcoal