everything is back together with the new washer (washer size same in thickness and O.D. as original)

I thought the breech would socket into the hole in the valve as well, however, upon inspecting the stock o-ring..there is no way the o rings I.D. will fit around the O.D. of the breach.

the breach just sits in top of the gasket (sandwich fit). This is one of the reasons I want to be sure the new nylon washer was the same thickness. I actually left it a BIT thicker as to get more pressure in the "sandwich" between the o-ring and the breach.