Diane,
I believe Ian (in alt.autos.gm) touched upon this not long ago.
Modern disc systems have drum brakes inside a hat assembly which serve as
the
parking brake.
I am led to believe that there is a Tech Service Bulletin out on this with a
new
procedure for preadjusting the parking brake.  Also, new shoes are
recommended,
I believe.

The shoes are supposed to be self adjusting, once installed correctly and
preadjusted.

I know this is not what you asked for, but I dont have access to the TSB.

I picked up Ians post, and with due credit to him, you will find it below

***********************************
""On this particular vehicle, the "park brake" (contrary to popular opinion,
this
is not considered an emergency brake, and if you attempt to use it as such,
good luck) is the drum in hat style...ie: it's a one piece shoe that sits
inside
the hat of the rear rotor.

Also contrary to popular opinion, there is "no" external adjustment on the
cable.  You will see a threaded end on the cable, but you will notice that
the nut is threaded onto it as far as it will go.  This is not where you
make
your adjustment.

In that year.....the park brake shoes had a design problem that allowed them
to stay cocked over to one side and basically wore the shoe material out,
even if you were not using the park brake.  The new shoes will come with
a re-designed retaining clip that allows the shoe to "float" and center
itself
properly after it is released.

Best bet....remove the rear rotors, check the lining of the park brake
shoes.
If worn....replace both park brake shoes, adjust them manually until you
can just slip the rotor over them, and you will have a decent "park" brake."
*********************
There is a manual adjustment for these, as he says.  I said they should be
self adjusting after preadjustment, but I am not sure,. now , that this is
true.

I went to a forum where this subject was being discussed, and it seems that
this system is just about as problematic as some of the early four wheel
disc
systems they had.

No real adjustment after the initial install. You replace the shoes and
the rotors and adjust them using the screw adjuster on one end of the
shoe bracket. Once adjusted so the rotor barely slips over the shoe your
done. No other adjustments possible.
Oh GM says replace the shoe when the lining is less than 1.5mm.
also the adjuster screw shouldn't exceed 5mm of exposed thread.