Lets start with the easy stuff.
Big Tube on filler and small tube on filler.
Big Tube is for filling. Small tube has two rolls. Let air out when filling and number two drain overflow from big tube back into gas tank.
Notice there is a bit of rubber tubing separating the filler neck metal from tank. This serves two purposes.
Safety is number one.
The gas tank is grounded the filler neck is NOT. if you try to measure resistance between outside of your filler neck and gas cap and your truck cab it should read null no connection at all no circuit. You don't want the spark on a empty vapor filled gas tank by a static ground or a electrical circuit touching the filler neck. that makes for a really bad day.
Number two is cab will move around a little and flex. you don't want that flexing you pipes causing leaks or other issues with your tank.
GAS CAP is vented both ways.
When you pull fuel out to engine it will suck air in so you don't have a vacuum build up and cause engine to die or worse catastrophic failure of your tank imploding.
Gas Cap will hold some pressure but ones it reaches a certain point will vent out. This varies by model.
Modern cars/trucks with EVAP systems have a sealed gas cap.
They have a separate hose running from tank / fill to a charcoal canister. There is a separate hose from charcoal canister to your intake manifold vacuum with valve. There is also a valve on the ambient air hole on or piped to cannister.
When you first start your engine the computer commands a close of the ambient air valve this seals the whole fuel system. it then measures the pressures as the vacuum is pulled on system. if it detects a leak it notifies the computer that there is a leak and turns on the check engine light. if no leak it reopens the ambient air value and it stays open all the time even when parked to let air and in off the system as needed thru the charcoal filter. Good thing for environment etc. Straight venting bad etc.
while running the valve on intake manifold opens and closes to pull fresh air thru the charcoal thus purging fumes back to engine to be burned up. this make the cannister last a good long time. (living on dirt roads is a different issue as the the dust will get in charcoal cannister).
LS swap guys. (I have a LS in my 56) Turn off all this checking and extra plumbing. or do like i did and leave all the plumbing to keep the fumed vented gas going thru a charcoal filter. Just not doing all the leak checks.
