On Series 1's it's built into the air-cleaner lid.

 

How did you fix it? I used to have the same problem in my car, that is, the overpresurized gas tank (no question about it, the hiss was from gas getting out, not sucked in). I didn't have flooding problems though. And now I noticed it doesn't happen anymore (the overpressurized tank problem), even though i didn't particularly do anything to fix it. The only differences I can think of is that when I was having that problem 1) I was living in a different geographical zone, the mountains range in New Mexico at 8,000 feet of altitude and now living in California, don't know if that would influence the phenomenon, and 2) I was running a defective cooling system that was making the engine consistently go up to and beyond half the dial in the temp gauge. Now it usually stays on one third and rarely reaches 1/2, thanks to new radiator and thermostat.

 

On the underside of the Carb Air Cleaner Lid!!!
If bad you have to replace the lid. Or get a 81-85 style cannister which mounts on the passenger side engine bay tower.
sgieldon
steve