Air leaks out of FX Boss when filling from empty

I know I filled my bottle from empty before and I had this issue. There's a post somewhere but I can't find it. So I replaced a bad bottle gauge on my FX Boss and had to bleed to bottle to empty. Now I tried to fill the gun back up with a Joe B slow valve tank and the air is blowing out of the barrel. I know I've dealt with this before. Please, somebody refresh my memory on how to fix this. 
 
Boss have a free
floating hammer that doesn't have constant hammer spring tension on the valve.

one quick check with no pressure in the gun, in the side of the action there is a hole that you can see the valve stem blocking half of the hole, use a small Allen and push it forward you should feel the spring tension cycle 2-3 times, the valve movement should be smooth with a return snap of the valve spring.

by doing this you will find out:

1. the valve isn't bending and movement is smooth and fully close.

2. If you can't see the end the of valve stem The valve is stuck open unable to go back to the bore---this happen when pressure is to low when fired 

still leaking after blasting it shut?

the metal valve seat oring is leaking, the delrine or metal seat to be resurface.