Some of the fuel tanks have an anti-siphon ball on the hose, where it leaves the tank. I've heard of these causing problems when doing fuel work. You may want to check there.
Also - some canisters have a bleeder button/screw on the bottom. Have you tried that?
If one day you're asked:
" How did you spend your time here on Earth?"
Will you say:
"I kept a crabgrass free lawn"
It's time to SKI.