Drippy copper union...

File under - if it ain't broke, don't fix it...
I have a pair of 1/2" copper unions for my hot water recirc pump that now leak. They didn't leak before I loosened them to add a pair of ball valves and a swing check valve. One makes a drop about every hour and the other is a drop every 10-15 min. Niagra Falls it is not...
They have been thoroughly leanded on with a pair of wrenches w/o pipe dope on the threads. They were hand tight when I soldered the pipes around them. I am thinking I need to reflow the solder while they are wrench tight.
Any thoughts?

From my limited experience, unions work well the first time, and not great from then on. To make the seal, they are (slightly) deformed. If you get the thing back exactly the same way, it will seal, otherwise, you'll have a slight ring crossing the original one, and offer a leak. Maybe a little oil on the threads and crank down a slight amount more?

Had a similar experience the other day trying to tighten old, leaky 1/4" union on a roof feeding into swamp cooler. Ended up tightening it down so much I split the darned nut. Then I had to replace the union.

This proves that I'm not a pro plumber, but I've been doing plumbing for about 50 years (began as a mere child of course...lol).
Whenever I have or expect a drip on a re-used union, compression fitting, etc., I just smear a little dab of clear silicone caulk around inside it (not necessarily on the threads) before re-tightening it.
Works every time.
Good Luck!
Mike

Try putting a lite coat of teflon pipe dope on the 2 union faces and on the threads, then tighten it again. Then let us know what happens.