Maybe I am misunderstanding here, but are you talking about the old steel single line tanks? They have 3 positions for the cap. Closed, vented, and open. If you close the cap all the way (to the right), then turn it left one "click" it will be kinda loose and sloppy, in a middle position, letting air in, but you cannot lift the cap off til you turn it all the way left one more click. Run it in the middle position, close when finished. Try that on your cap you will see what I mean. Hope this answers your question.