there might be a leak anywhere in the evap system.
see if it builds pressure in the tank after some time. if not you should inspect the line that comes from the tank and goes to the evap canister. after it goes to the canister, it goes thru a solenoid to the intake manifold. the evap line is usually made of metal and comes out somewhere around the firewall. check a manual for component locations.
do you smell raw fuel? the fuel supply and return lines can cause depressurized tank if air is introduced to the system. check for rust spots on the two lines.
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