fill nipples have an insert in it , so when you put the air in it goes down to allow the air in, but slips up and closes with the pressure of the air, to stop the air escape,
this has an "O" ring on it and sometimes need to be changed if the air comes out of the nipple due to mud ect, ( that`s why you need to have the Nipple`s covered )
If you haven`t put any air in yet then you just need to put a small amount of air to seal this to find out if the Nipple you`ve fitted is ok, 500 psi should do it,
I`ve around 30 x alsorts in my case and fitted way over a 100, you shouldn`t need to worry mate, but there again I`m not there to see what you`ve done !!
Tip = have a little spanner handy , to tighten , just in case you haven`t screwed it in far enough ??