I have had the same problem when I first started to use this valve. I was replacing a BVM retract and brake valve during a refurb on my Viper. The system was working perfect with the BVM valve, so I thought. So I sent Xicoy into DreamWorks, and they replaced it under warranty. The second valve arrived and I was having the same issues. I thought I got another bad valve. Because assuming the new valve wasn't working again, since with the BVM valve everything worked. I then started to seriously looking into my air system. Come to find out that I had 3 leaks in my system that were harder then H3ll to find. One of the hardest to find was on a retract nipple that was safety wired too. At some time when I had replace the air lines on previous maintenance, I did a no-no and used an Xacto knife to cut the previous air hose off. The knife scored the nipple. With the hose safety wired, there was still a micro leak. This leak was enough to make me think the valve wasn't working properly, when in reality, it was my system. I am telling you this, because I would have bet money on my air system being tight. Just because it worked with the BVM valve. The Xicoy valve is sooooooo sensitive that any micro leak will cause it to not properly work. So, triple check EVERY connection with soapy water. Just because it is safety wired, doesn't mean that it won't leak. The valve has worked flawless since I found all of my leaks.........that I would have bet money on weren't there. The hiss you hear is the valve releasing the pressure so that there isn't a vapor lock to move the cylinders in the opposite direction. I just bought 2 more of these valves for my next jet. Good Luck!