Pressure in new tank- found out why

67 A bodies have a tube on the top of the filler neck that goes to a U shaped hard line tube that follows the filler neck down through the trunk floor and terminates open to the atmosphere inside the rear driver's side frame rail.

The extra tube on the sender is for later years return line from the fuel filter with 3 tubes.

The mechanic tied the return inlet to the vent on the filler neck thereby eliminating the tank vent.

Without a tank vent 2 things will happen..


The tank can collapse from the suction of the fuel pump. ( I know it's not the suction it's the weight of the atmosphere pressing on the outside and with a lower pressure inside, the tank collapses)

And...

As the fuel expands as the temperature increase and the tank without a vent builds pressure.

Stock 67 filler tube.




Stock 67 sender (the sender internal shape is unimportant for this conversation)


Later model sender with return




Have the mechanic properly cap off the return (it will be submerged in fuel when the tank is full)

And reattach the proper tank vent.