years ago my local plumping supply sold what looked like a soft plastic egg to push into the open copper pipe to prevent water from messing up while you solder then heat that spot about 4" into the pipe + it melts allowing water flow. dont remember correct name but no-sweat or similar came up but cant even find to mail order! there was 1/2 + 3/4" sizes. love to get some as girlfriends well water causes leaks regularly. i know it should be replaced with 3/4" plastic for domestic water. thanks in advance for a working source