Review on Convenient Self-Sealing R134A Can Tap Valve Refrigerant Dispenser: Versatile Brass Valve Tool with Tank Adapter for AC Freon Charging Hose (Blue) by Chris Palmer

There is no user manual for this product. While it may not seem difficult to use, it isn't. Make sure the cylinder outlet valve is fully retracted into the housing, otherwise refrigerant will be released from the cylinder when you screw the valve assembly onto the cylinder. Also, you break the brass fitting and you end up with a useless tool. Piston valve outlet does not rotate when inserted. I did exactly that because I was guided by the words on the valve and not by looking at the valve itself. You need to make sure it's fully retracted before screwing it onto the can. Check it out and twist the valve back and forth and you'll see what I mean. I didn't blame the seller because it was my fault but a simple paper telling you this would have avoided the problem. I ordered and received a replacement at my own expense. It appears to be a quality item and comes with an adapter allowing it to be used with a range of standard calibers. It also comes with a nice long valve stem puller. If you buy it, be aware of this issue.