I went to replace my rear brake lines and can not find a line that goes from 3/8 to whatever size thread that the rear wheel cylinders are. Any suggestions, I already tried a 3/8 female to 1/8 male adapter.

 

parts stores should have lines with different sizes on each end that you can bend, if not they should have the nut you need then you'd have to cut the line and flare it

 

I bought my Silverado new and seldom drove it in winter. Yet my brake lines had total failure in 2009 with 48,000 miles. I took to the GM customer service did nothing please suggest me in this issue

 

You need to call around to some Auto Parts Stores (including Chev Dealers) and see if they have Steel Brake Lines of the diameter and length you need, to replace both sides. They will already be reverse-flared and have the proper size nuts installed.

DO NOT try to reverse-flare and use Copper Lines for this. Copper is NOT RATED for the pressure the MCyl will develop, and they WILL FAIL ! ! !

In my area, a store called Auto Zone carry Steel Lines, you might try them if they are where you're at.

PS: I think you are trying to identify the size of the line by what size you think the nut is. Most line sizes are 3/16 to 1/4". You probably should take one of the old lines along with you, and just match it. They're the same diameter (not length) on both sides.

 

X2!!!

 

The line could be made however you can contact classic tubes they will send you a pre made set that fits perfectly.

Classic Tube, Preformed Tubing

 

Thanks for that link !
I don't know why those Steel Lines are so scarce. That shop I mentioned is the only place around me, that carry's them.

BTW OP:
If you don't want to wait on shaped Lines, then don't worry about getting specific "Chevy" lines. Numerous types of vehicles (Ford/Chev/Dodge) use the very same stuff. I think that someone like Bendix makes the lines and slave Cyl for many manufacturers. I've used a 3/4" piece of Pipe to roll the bends, and a old Rubber Hammer, to shape the line around the Diff.
Do think about running the Line on top, or just behind your Diff, specially if you go out in the boonies. Most times hazards move from the front to the rear (underneath) your vehicle. Those heavy duty Tie Wraps work good for clamping the Lines down to the Diff.