I have a 70 Challenger(non ECS) with a 440 six-pack that has just been through a rotiserie resto. After starting the car and running it up and down the driveway a few times I noticed a steady noise coming from the fuel cap. I removed the fuel cap and it relieved alot of pressure. The noise is apparently coming from pressure leaking through the cap seal. I tried to blow through the cap, but could not (yes, it is a "vented"cap). I drilled a small hole in the plastic check valve and could then blow through it.
Problem solved , right?
On my maiden voyage accross town to attend a car show I arrived and all was quiet, but there was now a couple "brownish" fluid stains from the gas cap to the wheel well that I could not wipe off. Obviously gas decided to leak out of the cap and mix with what I think is sound deadner material and then streak my new PPG Clearcoat.
Every thing is new on the car, gas tank, fuel lines vapor seperator.
Did I just have a bad vented cap? Is there something else I can be overlooking. Why is there so much pressure in the tank?
Any help would be appreciated.

I would've guess cap, but since you've checked that. Does it have the cannister for vapor recirc? You might unhook one hose at a time and see what it does then.

Also, would it have a roll-over valve on the tank?

I think the VENTED stuff on caps is just to let get in air when gas pump is suctioning, but not to release the pressure... I'm having the same deal, with some whistly sound when pressure is too high, throught the gas cap gasket

I have had cars that gave out a big whoosh when you took the cap off to fill it & ran excellent. It had no problems other than the noise? I would suggest leave it or try another cap which you have to do now as the drilled hole will continue to dribble out when it's close to full & esp when you turn right.

My filler tube has no nipple for a vent line (never did). I seem to remember that 70's were different than other years. If the vented caps only let pressure (air) in then how is the pressure relieved? If it is through the tube as shown in your pic then why wouldn't that same tube let air in thus eliminating the need for a vented cap? Do some years/systems run with tanks at atmospheric and some under pressure? The body shop manual I have shows a vented cap for 70. I'll try to copy that section tomorrow and post it.

With the noise I get from this cap I could join a band! You may be right, could be a bad cap.

No cannister, I think that was for California cars only. Manual shows vent off the vapor seperator returning back to the sending unit. I have never heard of a roll over valve on a 70 although the sending unit is new.

I know the 71's(late70's?) had nothing vented to the atmosphere due to the very strict gov't emission regs of that era & the tank was vented with a hard line up to the charcoal canister on the pass fender & fumes went from the canister to a (sealed)3 nipple breather cap on the pass valve cover then down into the crankcase then over & up to the PCV to be burned.

The vented caps vent to outside air for pressure and vacum. When hot fuel cools it will pull a strong vacum so you have to vent under a vacum and with pressure when the fuel heats up and exspands. The newer cars that are sealed caps still release pressure and vacum at certain pressure and vacum settings as a safety feature in case the vent lines get blocked. I would replace the gas cap first with a good vented cap and see if that does it. Make sure it is a vented cap as I have seen guys think they got a vented cap and did not. Its not unusual to hear a little noise from a non-vented cap when it releases past its safety setting but vented caps dont usually make any noise because it is always vented and should not build pressure or vacum. Ron

I guess there are 2 types of systems in 70, I believe the early builds had the gas vent to the atmosphere, they later changed to the recovery system.

Here's a clip from the FSM.