It seems like gas caps should be working like a manifold thermostat except the spring allows it to open/close based on pressure versus heat. I’m guessing the spring specs for the cheap copycat caps aren’t being adhered to or are wrong. Removing the springs and drilling holes is akin to a wide open (well, narrowly open) gas tank in your garage.

1957 Chevy 3200
Daily Driver
PS, A/C, Tilt column, Rebuilt 350, Rebuilt TH350, Reupholstered Bench Seat, sound proof/insulated, LED headlights/taillights/backup lights/interior courtesy lights.
Follow in the DITY