Bicycle Mechanics - Importance of Brake vs. Derailleur Housing in Specific Instance

I am in the midst of rebuilding an early 90s Diamond Back Master TG road bike. I have come to rebuilding the rear derailleur system. The bike is equipped with downtube shifters and the cable only had housing in a loop from the chainstay cable stop to the rear derailleur. This is the only bit I need. The rear shifter is selectable between indexed and friction. Frankly, is derailleur-specific housing absolutely necessary in this application or would brake housing suffice?

I question the importance of derailleur housing in this application due to the slight movement of the derailleur and after reading somewhere about how the helical strands of derailleur housing really only come into play as reducing housing compression when in frequent-movement application, i.e., from a bar-end shifter to a downtube cable stop.

Thanks in advance!

Cheers

Go with the der cable. It's not that expensive and it does take quite a bend at the rear.
Never use der. cable with brakes. It is not meant to stand the compression.

for that little bit of cable, the leftover brake cable should do the job perfectly. Just make sure you get the length right. The fact that there is a bend there is not of major consequence, since it is for all intents and purposes a rather stationary bend, the radius isn't changing much as you shift throughout the cogs.

Some older bikes just used a length of coiled spring without a plastic coating on that segment. It looks cool, and it works just as well, but it's completely unsealed.

If it was my bike I'd use the left over brake housing but I'd leave the shift cable a little long. That way, if it doesn't work, you can buy the "right" derailleur housing to use later. Leave the shift cable a bit on the long side so you have some extra to cut off if the end frays.

Using the brake cable will mean a sloppy fit for the der cable in the housing. Of course, you would be saving the $1.50-2.00 cost of a foot of new der cable. Logic thus dictates you use the brake cable. The savings are just too great to overlook. bk ;););)

$2.00 at most, and after tax!

Still, with just a 7(?)-speed rear and shifter set to friction mode, it might work just fine. I wouldn't do it with any new bike.

Linear bundled wire housing came in with Index shifting, if you want it to Klick shift, that's the way to go..
the top pulley float on Shimano index shifters allow a little mis allignment

Coiled square wire housing was fine for both before that, when precise shifting was a skill rather than a gadget.

Still use that stuff for Rohloff shifter cable sets because its got all the gear sequence selection in the hub.

Thanks for all the replies, guys. I think I'll go ahead with the brake cable for the time being I was just looking for some other opinions.

Cheers