They will inevitably be corroded. To test for leaks,plug the end and do a pressure/time check.
Supply lines
Use washing machine hoses instead of plastic supply lines.
Supply is above and return is under.
Vacuum lines supply vacuum for various functions.
The Union, or the North, was able to maintain better supply lines during the Civil War. This was due to the fact that the Union had a larger supply of railroad lines, allowing supplies (such as arms, clothing, food, or soldiers) to be transported to the front more quickly.
we have textbooks
no....never use a solvent when working gas lines
U-boats
Supply, return, vent.
If you are asking whether it is possible to upgrade from single phase to two phase, then you should understand that a two phase system is archaic and will not be available from your electricity supply company. If you are asking whether your company can supply two lines (out of three) it's quite unlikely -they will normally supply single phase or three phase! You can likely upgrade to three-phase if you have a reason to do so -e.g. to operate three-phase motors. As for the cost -you'll have to check that our yourself!
THERE IS USUALLY A VACUUM AND BELT DIAGRAM IN THE ENGINE COMPARTMENT. IT IS SOMETIMES CONCEALED BY A REMOVABLE COVER OR FAN SHROUD.
they can, especially the smaller lines from a water softener, I think you need to get more specific