The older plumber left a over tighten copper pipe (threaded) to the shower valve. I tried but couldnt remove it with wrench. I dont want to use too much force as it may break the valve. I wonder if there is anything I can apply to it to help loosing it? Like WD40? I can see the teflon tap wraping around the thread and so it shouldnt be this hard.
First, check the basics:
- Are you turning the right direction? It is easy to get backwards when you are in an unusual approach. Inspect the thread closely for the correct loosen direction.
- How long is the wrench? An 8 inch wrench is probably insufficient for loosening 1/2 inch copper fittings, though it probably is sufficient for tightening them. Try a 12 or 16 inch wrench. The greater leverage of a longer wrench provides greater torque, but more importantly provides more control at that torque so you won't be as tempted to "pulse" the torque.
To protect the valve and safeguard other plumbing, be sure to provide counter-force at the other side of the joint with a similar wrench or vicegrips, etc.
In my experience, WD-40 is no help with plumbing.