How do you replace the plastic lines?
 

I can easily spot and replace dryrotted rubber vacuum lines on my car, but how do you replace the plastic vac lines (and some may be for stuff other than vacuum). They've become brittle and can break easily. My mechanic said they're hard to replace because they squirrel around and you have to find where they run to, and when you try to pull them out, they break.

I've looked at them in days gone by, and it's hard to follow where they go.

Any advice on this relatively simple repair?

1991 300d, 196k

I have an 81 300SD and all my plastic lines are original. In my experience they just do not get brittle and break. I would suggest that you just buy some rubber tubing and wait for a break (use a Mityvac to isolate). Just cut out the inch or so that broke and slip the rubber tube over both ends for a repair. They are nearly indestructible and I am not sure what you mechanic sees.

The plastic lines can break in a hot environment like the engine bay, but its not any big issue, buy some line in bulk - this website also sell it, the only issue is that it comes in white color only.

buy some rubber vacuum tubing and mend the plastic lines as required. I repaired some lines on my W124 like that, and a few dozen other cars too. Some had entire pieces missing.

You can also use air brake tubing to replace if you want. You'll need to take things apart and perhaps remove the dash to get all. The tubing can be bought in different sizes and colors. It is nearly indestructible. I have some plumbed to a fuel pressure gauge in the Ram and really rough handling would be required to hurt it.

plastic lines in my 95 GMC diesel have all failed and I've replaced with hose or copper tubing but my 93 300D plastic hoses are in good shape. where they broke thru ham fistedness I used flex hose to patch and small zip ties to secure.

They're incredibly tough.