If my memory serves me, isn't there a rubber hose that goes to the caliper but the main lines are steel?
Yes. After doing research, it appears rusting of the lines is a common theme and would make perfect sense why it is happening on just one caliper. Is the best route to buy pre-bent stainless lines?Have you ever flushed the fluid?
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Buy better parts.
Buy parts somewhere else.
Find someone else to do the repairs.
Use a better technique (clean everything, & use the correct grease).