I think I know how brakes work but can't fathom what's causing this, so
I must not know enough...

On my '79 westie, the left rear brakes need to be adjusted every three
weeks, no exaggeration.  I adjust them up, nice and tight, and then,
little by little over the weeks, they get worse and worse.  Crawl back
under and adjust again, the left rear has loosened up.  I tighten it,
feels great, about three weeks later I need to do it again.

What in the world is going on?  I've done this four times already.  Only
think I can guess is that the adjusting starts are loosening up from
vibration, but why?