Not sure. What do you think AZ? Any others?
My first inclination is to leave it on since it seems to be sealing pretty well and it might keep gas from burping out of the filler on to the paint. But if there is enough buildup, it's going to eventually get past the cap and do just that anyway.
If you leave it open, it will surely stop the buildup of pressure (one would hope anyway) but may leave you vulnerable to gas spilling out during hard right handers or off-camber parking and such.
Neither is good for paint, but old cured paint can handle it for awhile. On new paint that would be very bad indeed.
I don't personally see how you can build up pressure if your evap system is functioning. So I would check that asap just to make sure first that the parts are still there to begin with, and second that they're working.
Is this a stock tank still? And aftermarket tank?
If stock, there should be two small (about 1/4" or so) vent tube fittings just in front of where the main filler opening enters the tank.
Two rubber tubes should connect them to bent steel lines running along your rear wheel well towards the front.
You should be able to see three tubes (for a single tank) or four tubes (for dual tanks) then running vertically into an oval shaped rubber grommet. On the other side of that panel would be the condensing tank.
Three hoses to the tanks, and one comes out, runs across the body brace under the seats, and up to the front passenger wheel well area where you would have a metal charcoal canister bolted to the frame rail there. Normally has a steel rock shield strapped to it.
If that stuff is all there, you may have to blow some gentle compressed air (maybe less than 10psi or similar) just enough to listen to make sure it is all flowing as it should.
If it's not there, or is not functioning, the quick fix is to create your own vent to atmosphere, perhaps using the part that was linked above.
If the venting works, you should not build up either positive or negative pressures. Your cap should actually help to keep vacuum from forming, but is not allowed to vent outward to atmosphere after '70 model year.
If you just use a simple venting system, you will still get fumes where you park it. If you park it in a garage or elsewhere that fumes would be annoying (or dangerous!) you should get the full factory EVAP system sorted and working as it should.
Paul
