Slow fuel fill

[William dawson]

2008 avalanche 5.3 very slow fuel fill found busted charcoal fuel canister replaced and dropped tank and isolated all lines and flushed all charcoal from lines above tank,line from fill nozzle and line which  goes to engine purge solenoid all clear now reinstall tried still same problem. Dropped tank again all lines clear what is the weird check valve at tank neck with spring it moves pretty easy? Also what is in fuel fill line between fuel filler neck and tank? I have also a couple months before this problem had engine code said bad purge valve at engine replaced code went away couple months later no fill problem cannot figure out.also I forgot I also replaced canister vent solenoid which is located under spare tire this did not fix fill either I have replaced every part except could the fuel sending unit vent line be clogged I put low air pressure in with tank removed seemed clear also I am at a loss any help would be appreciated thanks. I also have no engine codes but tighten gas cap message comes on once every 20 or 30 times I drive for first five minutes only ,then goes away till next time.

[riverbanks]

Check the 99-13 Silverado subforums,2days ago?tons of info