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(Click on image to enlarge.).
For your assistance.. this is the typical make up of the GM rear brake system on passenger cars with say 9.5 or 11 or 12 inch rear drums... ie all Firebird/Camaro/Chevelle/GTO etc..
This is actually the illustration from a 79-81 Firebird or Camaro rear brake drum.. but most all are designed in a similiar layout.
The Wheel cylinder (not shown) goes in the center upper area.