The ball valve you show appears to be a compression joint valve. ... I don't feel the compression seal holds up as well as a solder joint.
... be sure it is a full port valve. If not, your flow will be restricted.
... Gates last for a long time
Thanks for your useful comments. You are right, the ball valve has compression joints. The reason I'm thinking of replacing a soldered joint with a compression joint is that as a DIY, I do not have a soldering kit, and I saw a YouTube video on how to do the job with a compression joint
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It is a full port valve, thanks for pointing that out.
As you mentioned, the valve is used twice a year, and it was probably installed in the early 1980's. It is mounted upside down (the turning wheel is facing down), for space reason. Because of its mounting orientation there probably isn't any debris that stopped the gate.
However, it is showing some greenish/blueish build up outside the body of the valve, which I believe happens when the material of the valve is exposed to water and air simultaneously. So I tend to believe that the gate stopped closing perfectly due to some of the same buildup. I was wondering if just opening the gate valve and cleaning the gate would fix the leak.