The gas cap is supposed to hold pressure in, no fumes or vapors are supposed to get out through the cap. It will let air in, somewhat, but not out. Fuel will expand during driving, heat from the exhaust, the pump, vapor pressure,whatever,,it will expand. The expanding vapors are supposed to escape thru the vapor recovery tube and be absorbed by the canister. Whenever the engine is running the canister purge valve does its trick and engine vacuum is applied to the purge line,,sucking the stored fumes from the canister, and some fumes from the line and tank. Problen is,, normal engine vacuum will quickly collapse the tank. To prevent this, there is a small restrictor tube in the purge line that restricts the amount of air being sucked from the line. The gas cap will allow some air into the tank remember,,the restrictor tube is to prevent more air being sucked out than the cap can allow in. The restrictor tube is inserted into the gas tank purge line along the bottom of the radiator. Just a small piece of plastic tubing. The hole inside it is less that 1/16". Make sure it's open, but don't enlarge it. If it is plugged you will get a pressurized tank, if the hole is to big you may collapse the tank.
Perry
