why can't I weld copper?

Do you want to braze copper, or weld copper?

Both can be done. But I'm not understanding your question?

You can absolutely braze over a pinhole using 15% silphos.

If you scuff just around the pin hole, and you add a tiny bit of flux right at the hole, you can usually blob some solder and stop the leak. This of course is a temporary solution but will buy you time, but to answer your question, you need the right brazing rod. I still wouldn't trust it - one pin hole often means there's another close by.

Hi, I'd go with the 15% silphos. Use mapp gas. I repaired many freeze damaged copper solar collectors using this approach. More or less heat affects how the filler metal flows, so you have a lot of control. I do like to clean the copper thoroughly first.

Yours, Larry

As far as I recall brazing is allowed for buried pipe soft soldering isn't.

A flare works but a pack joint or similar coupler would probably be the first choice most would make.

As far as brazing copper it's done all the time especially in refrigeration you can just about fix anything if you can get to the leak. Coils are tough to repair first you have to cut the fins off and the tubing is usually very thin so too much heat and it will disappear in a hurry.

As far as using it on water it will work but I don't know if brazing rod is lead free.

Does solder have to be sanitation approved?

And as @GroundUp up mentioned you can tig copper.

If your question is, "why is it prohibited" and not, "is it possible"? The answer is a contamination concern. On a water main, underground, a pin hole is what the backflow people call aspiration. Aspiration on a pipe can cause the venturi affect and with flow changes, turn a pinhole into a suction port (very basic description) and draw ground water into domestic main causing contamination. For a long time soldering underground at all was prohibited (not so anymore I believe) and brazing was supposed to be inspected (dont think so anymore either). With flare unions and water service locking compression fittings, its literally never necessary. I have had to braze repairs on soft copper that supplied gas lamps, but thats fuel code, not plumbing, dont think its at all addressed in the mechanical code.

https://youtu.be/IBxEpB4ZqRs

I think you would need an actual venturi for any flow to be pulled into a pipe or tube under pressure?

The venturi creates a low pressure zone, this is how a carburetor works, the soap nozzle on your pressure washer, or your aquarium vacuum.

Backflow or backsiphonage would be need to create a negative pressure to pull water in. Which could be possible with a broken main or large fire hydrant draw up stream.

silly question

Did you get all the water out!