It's time to go back to basics.
First let me say - this is solvable. Don't lose heart.
Second if there is a
significant vacuum in the tank it is a "venting issue" of some sort.
If it wouldn't start after you relieved the vacuum; then I would guess fuel delivery to the carb hadn't re-established. That is - the carb had lost prime.
Let's start here.
1. Pull the tank vent. Can you then blow air into the vent by mouth to verify it is allows air in?
2. Can you verify nothing obstructs the venting
passage of the tank itself? IOW if the vent itself is good; then let's move up the line where the air goes next - to the inside of the tank.
3. I'm wondering if it's possible that the tank and vent work fine and that the fuel delivery problem is a weak or perforated carb diaphragm.
If we assume for a moment that the hissing is "normal" and very slight; then assuming your fuel lines are good perhaps it is another component such as the carb (diaphragm)
Don't give up the ship.
With the help of the knowlegable folks here you can get it resolved.
I have great faith in the AS core members.
Have a look and post back.
-br