When soldering a copper pipe i had to stop a leak ...

I think you're going to have to open the line and try to flush it out with a blast of water from the other end. When you're soldering pipes and need to block water so that it doesn't settle into the area you're trying to solder (preventing the pipes from getting hot enough for the solder to melt), an old plumber's trick is to stuff a little white bread into the pipe to block the water long enough to make the connection. When you turn the water back on, you can then open the nearest valve to flush out the bread. I wouldn't do this up-line from a water filter or ice-maker or anything else where the bread debris could cause a problem, however. You can find more water supply repairs here: