Although the rear brakes should adjust themselves, they don't always adjust properly.
As the brake shoes wear down, the auto adjusters move the brake shoes closer to the drum
so the foot brake travel isn't increased but these adjusters can seize with brake dust.
Eventually the rear brakes may not function at all, this will be evident by a non functioning handbrake.
At this point you can either fit new brake shoes or adjust the handbrake.
Adjusting The Brakes
Thanks to Chris Ryu for the information
First we need to ensure the brake shoes are wearing and braking evenly.
Jack the car up and remove back wheels. Undo the tiny torx screw (careful not to round it off!),
remove the drum and clean to avoid harmful dust entering the air. You should see a small gear.
As you turn this gear, you'll hear a click. This is the capture mechanism that forwards the adjuster on
in normal use and also stops the adjuster from traveling backwards which would leave you with no brakes.
Other sites tell you to adjust this wheel as it is but it makes it hard to adjust the gear back without 2 screwdrivers.
I'll tell you how these are supposed to work, not just tell you to jab screwdrivers in it.