The tank expands and contracts due to internal and external pressures placed on it. Gas expands much more than water in warm weather or direct sunlight, so the tank bulges as the pressure builds internally. As warm gas gets cooler it contracts, creating a lower pressure inside the tank than outside, so outside pressure pushes on the walls of the tank and gives the tank the appearance of collapsing. This also happens when an unvented tank is being used while running. As the gas is pulled from the tank by your fuel pump it creates a vacuum inside the tank and again, the external pressure of the air will push on the tank walls giving it the appearance of collapsing.
Looking at your picture I can see what appears to be a vent in the middle of the cap without the hose attached to it, where you fill the tank with gas. Open it slightly and it will release internal pressure when the gas expands due to becoming hotter, and allow external air pressure to enter as the gas cools, stopping the bulging and collapsing of the tank walls.
Like the others, I really have no positive knowledge of what that grey thing on the hose is, although it looks like a type of pressure regulator. My guess would be it's used to prevent pressurized fuel from an unvented tank squirting out of the engine end of the hose when coupling or uncoupling the hose from an engine. Early 6 gallon metal gas cans used a pressurized molded 2 hose fuel deliver system to deliver fuel to the engine. But the problem was, because it was pressurized, if there was a leak it would spray raw gas all over, creating a dangerous fire hazard. To solve that they went to a single hose with the bulb that we all know and love....but the problem with that system is if gas in the tank isn't vented, it can create pressure within the gas line, and when the hose is being attached or disconnected it can spray raw gas all over. NOT good if you are hooking up a second tank of gas to a hot engine. So I'm pretty sure that's what it is. And it's automatic, so you don't have to do anything to it. Just leave it alone and let it do it's job.
__________________
1987 V20 w/1987 150HP Yamaha on a Shoreland'r Trailer
1978 16.5 Airslot w/1996 120HP Force on a Four Winns trailer
1996 V21 w/1993 200HP Mercury on a Shoreline Trailer
All towed by a 5.7L Hemi Durango.
If God didn't have a purpose for us we wouldn't be here, so
Live simply, Love generously, Care deeply, Speak kindly.
(Leave the rest to God) 
Silence, in the face of evil, is itself evil. Not to speak is to speak, not to act is to act. God will not hold us guiltless.
Last edited by Destroyer; 12-31-2013 at 12:25 PM.
|