GM Service Manual Online
For 1990-2009 cars only

Inspect the rear brake shoes every 6,000 miles and any time the wheels are removed (tire rotation, etc.). Inspect the rear brake shoes for wear by removing the rear brake drum. Replace the rear brake shoes when the thickness is worn to within 0.76 mm (0.030 in) of the shoe or rivet head. Always replace the rear brake shoes as a complete axle set.