Master Cylinder Push Rod

Place Booster Piston Rod Gauge (J-39567) on master cylinder, with master cylinder gasket in place. Lower booster piston rod gauge pin until its tip just touches end of master cylinder primary piston. See Fig 1 .
Remove booster piston rod gauge, turn upside down, and place on front of brake booster assembly. See Fig 2 . On all models, clearance between master cylinder primary piston and booster piston rod should be 0.0" (0.0 mm).
If clearance is present, adjust booster piston rod length by turning adjustment nut at end of piston rod until rod just touches master cylinder primary piston.

Fig 1: Adjusting Push Rod (Gauge Shown On Master Cylinder)

Courtesy of GENERAL MOTORS CORP.

Fig 2: Adjusting Push Rod (Gauge Shown On Brake Booster)

Courtesy of GENERAL MOTORS CORP.