How do I drain the brake lines? One wheel at a time through bleeder valves?

Then how do I refill and bleed the air out of the system?
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Is it metal or rubber line that needs replacing?  What I've generally done is
just open the system up where it needs to be, with rag and/or bucket handy to
catch drips.   After whatever has been fixed it needs to be bled.   I found a
good way to do that, with a bit of old fuel line stuck on the bleeder valve,
as i can aim it into a container, rather than it dribbleing everywhere.
        Flushing the system is done by pumpping all the old fluid through.
 with one person pumping the pedal till the reservoir is almost empty, and
the other catching it, fill up the reservoir when its v low, and bleed till
it comes out clean, and no bubbles.   bleed all wheels, to get fresh fluid
all through the system.

:)