TM 10-3930-659-34 4-51. SERVICE BRAKE DISC REPLACEMENT (Con’t). NOTE If   pressure   plates   or   brake   backing plate    are    defective    or    worn,    both brake    backing    plates    and    all    six pressure plates must be replaced. 3. Using depth gage micrometer, measure brake backing plate (5) and each pressure plate lining groove (13) depth.  Depth must not be less than 0.015 in.  (0.381 mm). c. INSTALLATION 1. Soak three new paper washers (9) and new preformed packings (8) in lubricating oil. 2. Install three preformed packings (8) and paper washers (9) in differential case (7). 3. If removed, install three pistons (10) in pressure plates (11). 4. Loosen bleed screw (12). 5. Install three pistons (10) and pressure plates (11) in differential housing (7). 6. Tighten bleed screw (12). 4-190