I am sweating a pipe from a valve to a 3/4" copper thread. What would cause the the threads to pop off the pipe?
It is almost as though I used no flux. Can flux become contaminated?
I am not quite following. Is the thread that is on the threaded adaptor breaking off of the fitting? Are you trying to sweat the thread to the valve? If so, this is not the common, accepted method of sealing copper thread joints. Teflon tape and/or pipe dope is the way to go.
On a standard sweat joint, the first step is to sand the outside of the pipe, then use an ID brush on the fitting. Apply a thin coating of flux to both surfaces, assemble. Heat until the flux runs and starts to smoke. Start touching the junction of the fitting and the pipe with solder until you see it start to flow, then wipe the tip of the solder around the joint.
Sorry if you already knew that, but sometimes someone doesn't know all the basics.
I have used flux that is pretty old without problems, so I would tend to doubt that it is the flux.
With a hot enough torch, you can solder one side of a valve with water on the other. You can also get it hot enough that it starts leaking, so I would attempt to drain it.
Hope you get it worked out.