Not 100% on 68s, but think you should be venting thru the vent line....stopped up?

I have a new tank on my 69, no vent line, but have a vented cap....they only work 'one way; though , let air in....
if you dont vent the tank and have a full tank and parked in hot weather, you can blow your sending unit gasket....then you`ll smell gas in your garage, dont ask how I know!

anyhow...I added a tiny hole in my vented cap , for relief of pressure....drilled from one side ,a hole less than 1/16"...and then from the other side, not aligned...and made a small slit in the rubber gasket to allow the air to the 'pocket'...have not had any problems since...the reason for not aligning holes, just to make sure nothing spills out on heavy acceleration w full tank....