Brake Vacuum Booster Push Rod-To-Master Cylinder

The vacuum booster has an adjustable push rod (output rod, Bendix design only) which is used to compensate for dimensional variations in an assembled booster. The push rod length is adjusted after each booster power unit has been assembled in production. A properly adjusted push rod that remains within the booster after it was assembled in production should never require a service adjustment.

A booster that is suspected of having an improper push rod length will indicate either of the following:

• A push rod which is too long will prevent the master cylinder piston from completely releasing hydraulic pressure and cause brakes to drag.
• A push rod which is too short will increase brake pedal travel and cause a clunk or groaning noise from the booster.

If necessary, booster push rod length can be verified with a push rod gauge using the following procedure:

1. Without disconnecting the brake tubes, disconnect the master cylinder and set it away from the booster power unit. The master cylinder, must be supported to prevent damaging the brake tubes.

2. With the engine idling, gauge and adjust the push rod length. A force of approximately 22 N (5 lb) applied to push rod with the gauge will ensure that push rod is seated with power unit.
   
   
   
   

3. Install the master cylinder on the power unit. Gradually alternate the tightening of the retaining nuts to 21-29 Nm (16-21 lb-ft).

4. With the engine idling and the master cylinder reservoir cover removed, observe the fluid surface in the reservoirs when the brake pedal is applied rapidly. Some fluid movement should occur in the forward reservoir. If fluid surface movement did not occur, the push rod is adjusted too long and the procedure must be repeated to prevent eventual brake drag.