[mickmate]

I'm retro fitting one of our keyhole button caps to an Aurora right now. It has a small 1/8" breather tube in the side of the neck.
Don't our caps vent through the screw hole in the middle?
On the new Shelbys they have a horseshoe shaped tube in the fill neck that draws a vacuum off the tank vent (outside of the tube) by a venturi effect (inside of the tube) from filling it. Of course my detailed explanation doesn't address your cap venting question it pertains to filling.