Hi

I forgot to mention in checking the vapor canister you can confirm if there is a problem on one the one side going down there.
If the large vapor line doesn’t hold a slight vacuum with the engine off then is has a problem with letting air into the intake manifold.
This not not prove there isn’t a problem with the valve on top of the canister though.

The other smaller line causes the valve to open so if you apply vacuum to it then the large vapor hose it will vent the canister. You will want to check its ability of operation to be able to open and close regularly.

FYI there’s supposed to be carbon in the canister emissions system to absorb condensing vapors until drawn off during idles. So it can absorb some small amounts of vapor from the tank.
I guess it’s explains mounting it under bumpers shade or tucked out of site period.

Gas caps vent off any overpressure but before that, the canister is supposed get a lengthy shot.
Sometimes you might hear the gas tank sound off, in a quiet car that’s sitting in the sun or if the gas has been agitated to some excessive extent.
The return line is doing that while pumping.

Actually that holding pressure can be quite a bit higher than my mouth can apply but I can suck air through the cap from the protruding screw side fair easily.
You can inhale but you cannot exhale. 🤪 Sorry Bill C.

A drop in altitude will add air to a tank to let air in to get gasoline out but if you climb another steep grade it equalizes.
I can only imagine how an aircraft deals with those conditions.
I noticed on big planes they fill from under the wing too! 🤔 Nice piping?

Gas caps are simple devices, work quietly but are very important. More so on with new car emissions standards. That why they get monitored.
I have heard of a few cars getting traded in for a CEL, that was later found out to be only a gas cap. 🫣

I looked into Art’s pictures and I agree wrenching on the Volvo filter is best done with an impact driver.
It may not look like it takes that much to loosen or compress those copper washers but surprise, it does!
With it hanging down from under the car its a grappling challenge.

Phil