The Pump N Plug tool is made of strong, flexible silicone rubber able to withstand temperatures of up to 400 F.
The tube easily navigates elbows and Ts, so it's perfect for any job, and lightweight hand-operated pump fills a bulb with air to quickly stop dripping water.
You can view how the product works on the manifucture website:
or call 877-222-2667
Why not just use a piece of white bread or paper towel then solder 1/2 a union in the tubing using Hercules solder / flux 95 -5 ( lead free) and then your good to go as now you can solder the other half of the union on the tubing and use either a new valve or continue working then prior to tightening the union the flush out the bread /paper towel.
Or use a Turbo torch with a male adapter using a good quality flux,solder the male adapter even with a slight drip then let it cool put Teflon tape on the male threads and screw on a valve.
Or if the tubing is 3/4" (7/ or less use a copper compression fitting or Flare the tube end and continues with the job.
400 deg F for soldering is very close to the soldering temperature and with the newer types of soldering torches one can easily reach temperatures over $1,000 deg one mistake and I would imagine someone may have just destroyed the tool.
Also when soldering heat of course is generated and there is something called pressure temperature relationship where the water can turn to steam and create really dangerous pressures. IMHO
I have soldered , brazed copper tubing for over over 40 years and managed to do it with out buying a very expensive tool that I would possibly use once in every 4 or 5 years if that