Gas leaks out from under my fuel neck filler cap during hot weather with the car sitting rearward at an angle. Recently at a car show it was 85 degrees, the car was sitting on a street slope with the back end down more than the front. Tank was probably fairly full. After several hours with the high temps, fuel began leaking out from under the fill cap. It has the original filler cap type, with rubber gasket.
Obviously unlike modern cars with special venting design, when fuel expands under heat it's got to go somewhere. But did Volvo design anything in these PV's to accommodate that issue?
I had to remove the car from the car show. Quite embarasing!

You may have simply overfilled the tank, especially if you tried to pump a little more gas after the pump nozzle initially shut off. The PV/Amazon filler necks were off the back of the tank, not the sides, plus the gas caps weren't as high above the top of the tank as newer cars. Having the back end down with a full tank can create an air lock at the front of the tank. Air expands way more than fuel, so when the headspace air in the tank warmed up it had to push gas out first before the air could escape through the gas cap vent.

Could also be there's a problem at the filler neck. The soldered joints in the old PV & Amazon tank necks were known to occasionally crack, possibly aggravated by abuse from the pump nozzle being pushed down too hard or an accident, maybe even driving off with the pump hose still connected. Check for existing fuel stains on the outside of the neck and tank. If needed, try recreating the leak to make sure it's not coming from the base of the neck. Fill it to the base of the neck, loosen the gas cap then either jack the front up or park on a hill, leaving it overnight in cooler weather to see if any fuel is leaking at the base of the neck. If it turns out its leaking at a joint, the tank would need to be emptied, dropped and re-soldered with a torch, being extremely careful that there are no gas fumes in the tank. A safer and easier alternative would be prepping the leak area and using a two-part epoxy putty rated for use with fuel tanks.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now