gas cap question

[dzbowtie]

hey guys,

this may be a very simple question, but i have a non vented tank, which i have a vented cap. the question is, is this vented cap supposed to be loose, with a slight wobble? it is a locking cap. if i place it on the neck with the first 2 pronges, til it clicks, it is obviously loose enought to vent. but, then i can turn it about a 1/4 turn, like a radiator cap, till it tightens, but then i notice some pressure building up in the tank, and some slight fuel coming from the key hole. should i just leave the cap loose? thanks for your input.