How do I fix a leak at a decommissioned saddle-valve?
The proper name for it is a self piercing saddle valve. The valves are bad for many reasons, including the fact that they often clog and no longer allow water through the line, and they leak.



If you need a valve there to supply something like an ice maker or humidifier, then I'd cut the pipe, add a T, and put a proper quarter turn valve on the line.

To remove it, I'd just cut the pipe right on the piercing and install a copper coupling between the two cut pipes. The steps include: shutoff the water, remove the saddle valve (unscrew the two sides), cut the pipe, clean it, apply flux to the pipe and a copper coupling, fit them together, and solder the joint by heating with a blow torch and working solder into the joint from the opposite side. A skilled plumber should have the entire job done in under 30 minutes including setup and cleanup. For the DIY'er, you'll need the following tools and parts:


Copper pipe cutter





Fitting brush





Flux and solder





Blow torch





Copper coupling




Note, no affiliation or specific recommendation with any of the above brands, these are only sample images.