Gas tank vent question - assist please!

[Bilgamesh]

I have a 74 with stock rear tank only. The two return lines on the side have been looped together. The vent line that runs beside the main filler hose is intact. From the beginning the tank has been slow to fill. More recently it will hardly take any gas before it backflows. I figured the vent was plugged by rust or something, so took off the line to see if I could root it out. When I pull the line off, the gas will trickle out indefinitely in a small steady stream, until the tank is empty. In other words it is obviously not plugged, but instead seems like it is at the bottom of the tank and an is acting like a siphon when it is unplugged. In this situation, it seem slike the vent line will always have gas in it, that will not allow the air to escape, and thereby result in my inability to fill the tank. I notice if I unplug the other lines, the air rushes out and the tank fills fine. Does anyone know how this line system is engineered, and how I can clear it up? Is there a schematic diagram of the tank and lines in the shop manual?

Thanks

 

[oleguy74]

check vent line going up side of fill pipe .this part might pluged?on single tank versions a return line goes to carb.havent run into one of these but thats what the ford book says.calif. cars had evap sys,

 

[Viperwolf1]

Here are the drawings for 1974, not much detail though.