Does not and Will not leak.
I've fitted these cup seals to several gun's I have owned and will continue to fit them. I can find nothing bad to say about them.
THEY are a MUST if you use CO2 and recomended if you use HPA
They are a very tight fit on the Valve Stem so when fitting make sure you protect the stem from scratching. I use one of those rubber grip pads you get for opening bottle tops.
These seals really do work I would recommned them to anyone.