The O-ring I purchased at the local parts store. I removed the valve and took it to the store to match it up with an O-ring.
You do not have to remove the entire assembly, but because that was my first time and I had no idea what was involved, I removed the assembly because it is quite simple to. Detach a few hoses and I think 4 nuts and it's off.
To do it with the assembly attached, get about a 6-10 inch piece of coat hanger wire, cock the spring lever wide open, wire it up to stay back and then just remove the valve and replace the O-ring.
Be sure to check the bottom hose for leaks, it can get soft at the connection and leak as well. That is what I need to address. That bottom hose is so short, i am not sure if I can snip the end and still have enough left to reach the connection.