DO NOT EVER use these caps for plugging a fuel line, vent, unpressurized, or not. First, they DO deteriorate. Second they DO come off. One winter day, with a bunch of snow and ice on the hood, and already close to late, I started the (then) 340 Landcruiser only to have one of those pop off the PCV connection on the AFB, thus causing a huge vacuum leak which I could not get to right away.

Use quality hose and clamps and some sort of plug --if you use a bolt, use one long enough that it has an unthreaded area, and clamp up on the smooth section, or else cut off the threads.