I owe a huge thanx to Craig (68tpls400) who emailed me off list a pic of his original cap. Mine which I thought may be original clearly is not. He inspired me to tear mine apart, I discovered a little check valve disc and a rubber seat. I put 2 notches in the rubber seat and now it vents both ways.

Wish I'd taken it apart 2 yrs ago when I drilled out a brand new one.