It may be not the problem but I've found that new tanks that use the 3 line system with nipples coming directly form the tank (like hayes or great planes) I mean, not the tanks that have aluminium or brass tubes for the lines, comes with the hole for the vent line so restricted that I needed to redrill it. It happened for example that sometimes pressure in the tank was so high that the line from the pump was disconected and I got a fuel bath.

Hope this helps