I just bought a 1970 Javelin SST 360. I was wondering does anyone know if those cars typically came with
a vented gas cap? THe car develops extreme pressure and when taking the gas cap off it practically blows right off cause of the pressure. Is there a vent line or should I buy a vented gas cap?  

a little history... of smog equipment...

in 1970 most cars came with non vented gas caps  and carbon canisters on the gas tank vent...  it is possible that these hoses have gotten smashed over the years ... or someone has removed the carbon canister... and plugged the lines...

on friday i topped off my 75 ford van.... and parked it... on sat morning i went out and it is down a 1/4 tank...(down a quarter tank.. now reading 3/4 full)    the gas has a bunch more aromatics in it.    (stuff that evaporates)  so you might be just overloading the system...

i don't have out my early emission control manuals right now...  the emission controls are listed in there...  and how to test them without damaging them...  

In 1970, AMC Javelin came with a vented cap on 49 State cars, and non vented on Calif. Cars.
The 68-69's had a short vent hose that ran off the filler neck and was open to the atmosphere (no Canister), I think the '70 utilized the vented cap.
 I have a locking, vented cap on my '70 Javelin, the venting is allowed thru a little check ball/valve in the cap itself. The Stant online catalog has the specifics.