I'm having trouble with my. 70 B body building pressure in the gas tank to the point were it pours fuel out the vent line. I have tried a new has cap (3) of them all vented. Ran a vent line up in my trunk big loops with 3/8 fuel line and back out still build pressure.
Help this is killing me.
Thanks

I had a problem like that years ago on a brand X.
IIRC it was the fuel pump.

Yeah we're missing sumpin simple but I'm thinking either the tank is being heated (exhaust) or it's being pressurized (the pump as said). Here's a BTT for ya

3" TTI out the back and I'm running MSD Atomic but I was building pressure before the EFI.

It sounds like the vent line at the tank may not be at the top. Maybe a defective tank or the vent is hooked up to the wrong fitting. A pic of the vent exit from the tank may help.

Also some '70's were "closed vent" with a can in the trunk to separate the vapor and fuel and a line to the engine for the vent. By chance was your's originally set up that way?

With vent installed it should not need a vented cap... and vented caps sold today hold a fair amount of pressure before they "vent" so the cap won't help.