This is most likely a petcock or float issue, but some things to check:
Is the canister hooked up or did you cap the vent line? If you capped it your tank won't vent.
Check to make sure you didn't pinch the vent hose against the frame tube when installing the tank. I've done it myself.
Remove the tipover valve (plastic hex-head chingas under the tank that the vent hose to the canister attaches to) and run a wire through the tube to see if it's obstructed. I've never seen an obstructed tipover valve but make sure it rattles.
Is the canister hooked up or did you cap the vent line? If you capped it your tank won't vent.
Check to make sure you didn't pinch the vent hose against the frame tube when installing the tank. I've done it myself.
Remove the tipover valve (plastic hex-head chingas under the tank that the vent hose to the canister attaches to) and run a wire through the tube to see if it's obstructed. I've never seen an obstructed tipover valve but make sure it rattles.