Slightly Leaking Union

[Lzrsrgn]

I did a shower remodel with 3/4” copper and put two unions in between the main supplies and the rest of the work. I left the wall open and noticed a very small leak from one of the copper unions. The CW Union comes up vertically and makes a 90 degree turn for a short run into the shower valve. It is leaking ever so slightly from the top of the union. I tried tightening it with two pipe wrenches, but didn’t work.
Short of cutting it out/desoldering and replacing, are there any plumbers tricks I can use? I don’t think PTFE Or silicone lube will work based on where the leak is, as it is under pressure. I would like to stick with copper and avoid PEX. Thank you.

 

[Reach4]

Lube on the threads helps tighten the nut tighter with a given amount of wrench torque. And after the first link below, I would put something on the mating surfaces.

Do your unions have o-rings? If so, I would for sure use silicone grease on the o-rings.

https://terrylove.com/forums/index.php?threads/how-tight-to-tighten-a-1in-union.33291/ and
https://terrylove.com/forums/index.php?threads/how-reliable-are-copper-union-threaded.102801/
have some interesting comments.

 

[Sponsor]

 

[Fitter30]

Black and galvanized unions take one wrench hold it to the back side other wrench smack the union nut retighten. Brass or sweat copper dope the face of union use adjustable wrenches and hope. Any fitting stainless when joint gets snug stop for a few seconds then repeat till tight. Dope with nickel in it.

 

[John Gayewski]

Get some antisieze. Lube the threads and the male side of the union face so it can slide into a sealed position. When you take the union apart pull the nut up and look at how the faces match up without the nut to hold them if one side it tweeked a little try to tweek it back into a face to face match so the nut is just pulling the two faces together instead of trying to twist the faces to match up.