with new cap on and both vent lines open I still get pressure in the tank???
You'll sure as **** get SOME pressure in HOT summer weather. even with the vent tubes open. Gasoline is VOLATILE AS ALL HELL, and MODERN "gasoline" is based on heptane and hexane more than the older stuff, which was heptane and OCTANE. The older fuel had other stabilizing additions like white lead (tetraethyl lead) which also made it heavier, and less apt to ping or knock. Consequently, the stuff behaved better in hot weather, aside from poisoning kittens and children et cetera...
The tank pad should insulate your tank a little from incident heat from the car. I often have to siphon off a couple gallons from Gertrude's tank after filling to keep her from pissing valuable, flammable petrol from the vents during summer, and am strongly considering a charcoal canister to catch that ****, though I don't plan to plumb it into the fuel intake up at the carb, as my '83 Dodge has it.
You very well may have some crap floating in your tank, partly plugging those vent lines too. My tank is just over 6 yrs old, and also a 2 vent sort, so I relate to your blues with this. Its hard to score a GOOD U.S. made OEM gas cap also I've found. Bad ones abound, but GOOD ones?! I have exactly ONE to date..... though the locking cheapo plastic thing from VatoZone did alright for the year I used it.
Purge your tank, then see after a good check and cleaning what it does.