Nice write-up ddsmith!
I've had the pressure problem a few times myself,until I accidently left my cap on the bedrail at the station and then run over it when it fell off!
I think I might get a new cap and mod it like you show.
Questions;
My truck is a 3/4 ton 72. I think it has the vent pipe that goes up the passenger side of the cab, (it been along time since I've had that area uncovered).
Where does this pipe go?
I think somebody mentions a canister of some kind. What/where is it?
My truck has never had any type of canister that I'm aware off.