what kind of line pressure are we talking about with manual drum brakes? reason i ask is i recently bought some of the generic brake lines from NAPA and installed them on my '71 bronco (i know not chevy). I am an aircraft structural guy and we make hydraulic lines etc all the time. from what i would guess about this generic tubing the material seems to be 2024-0 (annealed). in aircraft application this stuff is considered to be good to about 500 psi. add to that fact, the stuff we use on aircraft is seamless tubing. this stuff clearly had a seam which i am sure does not help it in the strength department. so i am wondering if i have a time bomb on my hands. looking back on it i guess i shoulda just snagged some stainless tubing from work. might still do that just for peace of mind.

The brake lines sold at auto stores should be at least DOT approved, or they would be in a world of federal hurt if something were to happen. I would hope the line should withstand at least 2500psi.
Most 4 wheel drum brakes shouldn't see much above 1000psi even under severe fade stopping. 500-700psi might be the norm for a hard threshold stop with that Bronco, if I was to guess.

I've run NAPA and other part store soft tubing on many cars without issue. The 20' rolls that you can get from the dealer are even softer, and flare and bend very easily... I dont know all that much about metallurgy though. I never paid attention to the seam. I thought it was supposed to be seamless though... Guess I need to find the DOT regulations on that.

If it concerns you, by all means replace it with something you like. I dont like SS tubing on automotive applications because it work hardens, is not supposed to be double flared, and is reported to cause excess corrosion on the attached regular steel or iron parts (galvanic corrosion?).