The trick? A torch. Use the correct bender.

The inner integrity can be hydrualicly maintained by a liquid trapped in the line.

Maybe the tubing you are using is the problem? 5/16 is pretty easy to bend, and that Imperial bender should bend 5/16 to 180* with no problem. I've never used "double wall" steel line in an automotive application, and hardware store tubing might not be the same temper as "automotive" tubing. Buy some tubing from a car parts store and try that. Also, if you have to flare the tube, use a double-flare and ALWAYS REMEMBER TO PUT THE NUT ON THE TUBE BEFORE YOU FLARE IT! It sucks to make a perfectly bent line only to realize that you forgot to put the nuts on before you flared it!

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Filled with sand/salt and could get 45 degree

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I tried that before, seemed to help.

Like above,these will bend anything without collapsing if you use good tubing,stainless or steel makes no difference.The small one will bend 1/8,3/16 and 1/4 inch with no problems.Also,it will bend a shorter radius than any others I have or tried.If you can't find them (Imperial Eastman) at an auto parts store,most all commercial refrigeration suppliers stock Imperial.

Mine is like the Imperial in a earlier post, never kinked a line. One of the things I do is to put a little oil on the handle portion of the bender, the part that bends the tube around the radius.

the "Ridgid' line works well.

ridgid brand