An old mechanical speedometer cable or Bicycle brake cable in drill, be sure to spin the proper direction so it doesn’t come undone.
When using air pressure (Marty stole my first thought) use an air blower that has the syphon straw and introduce some ethyl benzene if it is old fuel turned into a schellac like material. Just realize that it is already really flammable and when you spray it with that tool you just aerosolized it and now you have a ready made bomb so the other end need a hose going into a container to catch the mess as well as slow the fumes coming.
Do NOT aerosolize the DLRO we spoke about in that other cleaning thread. That can get deadly dangerous in an uncontrolled way.
IF you use DLRO, only put it in a container above the unit and let gravity feed drain itself through the line and have the other end of rubber hose go into something like 2 liter soda bottle that you can easily put a lid on and discard without handling it much. Glove up well for that one.
Various acids are better enemy to rust but will eat the metal line as well…
Again use the gravity method. Remember to neutralize the acid afterwards.
Better than me explaining which acids as the debate of which is best and by how much,
What ever you do with this, put a clear filter on the output side that you can see as is plugs and change/ clean easily. Those screw together glass inline ones come to mind.
Then from there have it go to your 5 micron filter. This will keep you from plugging the more expensive filter so fast because no matter how clean you think you get it- it will have tons and tons of “no see um” sized garbage for years to come.
If you can’t out both filters afterwards and this line goes straight to the injection pump- you can just replace the line when you replace the pump soon because that is around the corner. So replacing pump might fall under excessive disassembly much more than replacing fuel line. Abandon original line and fishing in some new metal or sae30r9 rubber line would be far preferable over cleaning the old line.