Chevrolet Cobalt Service & Repair Manual: Adjustments

Brake Pedal

1. With ignition OFF and brakes cool, apply brakes three or four times, or until brake pedal becomes firm, in order to deplete brake booster power reserve.
2. Adjust steering wheel angle to highest tilt position and lock wheel in place.
3. Install brake pedal effort gauge tool No. J 28662, or equivalent, to brake pedal.
4. Release parking brake and start engine.
5. With engine running at idle speed, measure and record distance from brake pedal to rim of steering wheel.
6. Slowly apply and maintain brakes with 66 lbs. of force to brake pedal.
7. While maintaining 66 lbs. of force to brake pedal, measure and record distance from same point on brake pedal to same point on rim of steering wheel.
8. Release brakes.
9. Subtract initial unapplied measurement from applied measurement in order to obtain brake pedal travel distance. Specification Maximum brake pedal travel, measured with 66 lbs. of force to brake pedal, engine ON, and brakes cool, is 2.8 inches.
10. Turn OFF engine.
11. Remove brake pedal effort gauge tool No. J 28662, or equivalent, from brake pedal.
12. If brake pedal height and travel require adjustment, turn pedal pushrod and then tighten pushrod lock nut.

Master Cylinder Pushrod

Proper adjustment of the master cylinder pushrod is required to ensure proper operation of the power brake system. A pushrod that is too long will cause the master cylinder piston to close off the compensating port, preventing hydraulic pressure from being released and resulting in brake drag. A pushrod that is too short will cause excessive brake pedal travel and cause groaning noises to come from the booster when the brakes are applied. A properly adjusted pushrod that remains assembled to the booster with which it was matched during production should not require service adjustment. However, if the booster, master cylinder or pushrod are serviced, the pushrod may require adjustment.
There are two methods that can be used to inspect for proper pushrod length and installation. These are the gauge method and air method. Usually, if the power unit pushrod requires adjustment, use the power unit repair kit gauge. The gauge measures from the end of the pushrod to the power unit shell.

Gauge Method

Tandem Diaphragm Type
The master cylinder pushrod length is fixed and is usually only inspected after the unit has been overhauled. This procedure can be performed with the unit removed from the vehicle if a suitable vacuum source is available.
1. Assemble booster unit and install pushrod. Ensure pushrod is fully seated.
2. Apply 20 inches or maximum engine vacuum to booster.
3. Position gauge tool No. J-37839, or equivalent, over pushrod, Fig. 1.
4. Replace booster if output button length is not within gauge limits.
5. Install power unit and inspect adjustment.
6. Ensure master cylinder compensating port is open with engine running and brake pedal released.

Air Method

1. Ensure master cylinder mounting nuts are tight.
2. Remove master cylinder filler cap.
3. With brake released, force compressed air into hydraulic outlet of master cylinder. Regulate air pressure to value of approximately 5 psi to prevent spraying brake fluid from master cylinder.
4. If air passes through compensating port, which is smaller of two holes in bottom of master cylinder reservoir, adjustment is satisfactory.
5. If air does not flow through compensating port, adjust pushrod as required, either by means of adjustment screw (if provided) or by adding shims between master cylinder and power unit shell until air flows freely.
6. Connect brake lines and bleed system.

Fig. 1 Master cylinder measurement. Tandem diaphragm type

    Power Brake Units
    ...

    Description
    AC-Delco Tandem Diaphragm Type This unit utilizes a vacuum power chamber, consisting of a front and rear shell, housing divider, front and rear diaphragm, plate assemblies, hydraulic p ...

    See also:

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