Your brake system uses two kinds of fluid conduit: rigid metal hard lines and flexible rubber hoses. The hard lines are the long metal runs along the frame and body. Here is what brake hard lines do and when they need attention.
What hard lines are
Brake hard lines are rigid metal tubes (usually steel, sometimes coated or stainless) that carry pressurized brake fluid from the master cylinder along the frame to near each wheel, where a flexible hose takes over for the last moving section. Because they are fixed to the body, they can be rigid - and they handle the bulk of the routing.
Hard line vs hose
The difference matters: hard lines are rigid and run along non-moving parts; hoses are flexible and connect to the moving suspension/steering at the wheels; see brake lines vs hoses. Hoses flex and wear out from movement; hard lines fail mainly from corrosion.
Why hard lines fail: rust
The number-one enemy of steel hard lines is rust, especially in the salt belt - they corrode from the outside over years until they weep or burst; see brake fluid leak signs. A rusted-through hard line can fail suddenly, losing that circuit. Coated or stainless lines resist this far better.
When to replace
Replace a hard line that is visibly rusted through, leaking, or badly corroded - do not patch a rusty brake line; it is a critical pressure part. After replacement the system must be bled - see hose and line leaks. Keep the whole system sound with fresh fluid, quality pads, and hardware.