Fuel pump mounting, & return line help

Well I was pretty careful. I did it like this:

Drained the tank, unhooked the vent connection in the trunk, and plumbed the vent line "out" to a scrap length of tube.

Plumbed my MIG CO2 regulator to the sender outlet. Turned on the CO2 for a "flow," then went up and LIT THE FUMES COMING OUT THE VENT!!!!!!!

I just let the CO2 flow until the vent line went out, tried to relite it several times, then let the CO2 flow for about another minute.

I also continued a small amount of CO2 flow while brazing. Not so much as a "hint" of a pop while brazing.