My vac petcock wouldn't turn off 100% either, but mine would drip slowly, not run flat out.
I used to think there was a PRIME passage that got opened when the lever was moved, but I have been told since by the vac petcock tear down experts there ISN'T a special passage -- instead there is a bump on the plastic innards that actuates a spring dingus that lifts the diaphragm and needle assembly assembly when you go to Prime.
Now I trust vac petcocks even less than I did before as I can see a whole world of non-function based off that spring dingus and plastic bump that we never even knew about before. Knowing the vac petcock didn't work well was one thing, but knowing that its guts were designed by Micky Mouse is completely another thing entirely.
..... get a Raptor, for your sanity and piece of mind