Component Tests

Brake Booster 

1. Disconnect the check valve from the brake booster.
2. Apply the parking brake, start the engine and place the transmission in NEUTRAL.
3. Verify that manifold vacuum is available at the check valve with the engine at idle speed and the transmission in NEUTRAL.
   • If manifold vacuum is available, stop the engine, connect the check valve and continue with Step 5.
   • If manifold vacuum is not available, continue with Step 4.
4. Disconnect the check valve from the vacuum hose and verify that manifold vacuum is available at the hose with the engine at idle speed and the transmission in NEUTRAL.
   • If manifold vacuum is available, stop the engine, install a new check valve and continue with Step 5.
   • If manifold vacuum is not available, stop the engine, connect the vacuum hose to the check valve and refer to ENGINE SYSTEM - GENERAL INFORMATION article to diagnose the no vacuum condition.
5. Apply the brake pedal several times to exhaust all vacuum from the system.
6. Apply the brake pedal and hold it in the applied position. Start the engine and verify that the brake pedal moves downward after the engine starts.
   • If the brake pedal moves, the brake booster is operating correctly.
   • If the brake pedal does not move, install a new brake booster. Refer to POWER BRAKE ACTUATION article.
7. Operate the engine a minimum of 10 seconds at fast idle. Stop the engine, and let the vehicle stand for 10 minutes. Then apply the brake pedal with approximately 89 N (20 lb) of force. The brake pedal feel should be the same as that noted with the engine operating. If the brake pedal feels hard (no power assist), install a new brake booster check valve and retest. If the brake pedal feels spongy, bleed the hydraulic system to remove air. Refer to Brake System Bleeding .

Brake Booster Check Valve 

The function of the brake booster check valve is to allow manifold vacuum to enter the brake booster and prevent the escape of vacuum in case manifold vacuum is lost during sustained full throttle operation.

1. Disconnect the vacuum booster hose from the check valve.
2. Apply the parking brake, start the engine and place the transmission in NEUTRAL.
3. Verify that manifold vacuum is available at the check valve end of the vacuum hose with the engine at idle speed and the transmission in NEUTRAL.
   • If manifold vacuum is available, stop the engine, connect the vacuum hose to the check valve and continue this test.
   • If manifold vacuum is not available, stop the engine, connect the vacuum hose to the check valve and refer to ENGINE SYSTEM - GENERAL INFORMATION article to diagnose the no vacuum condition.
4. Connect the vacuum hose to the check valve and run the engine for at least 10 seconds.
5. Operate the brake pedal to check for power assist.
   • If power assist is present, continue with this test.
   • If power assist is not present, refer to Brake Booster Component Test in this service information.
NOTE: Do not remove the brake booster check valve from the brake booster in this step.
6. Stop the engine and disconnect the vacuum booster hose from the brake booster check valve.
7. Apply the brake and verify that there is enough vacuum retained in the brake booster for at least one power-assisted brake application.
   • If there is enough vacuum for at least one power-asssisted brake application, the check valve is functioning correctly.
   • If there is not enough vacuum for at least on one power-asssisted brake application, continue with this test.
8. Inspect the brake booster for any signs of damage that could cause a leak.
   • If any damage is found, install a new brake booster and repeat this test.
   • If no damage is found, install a new check valve.

Brake Master Cylinder - Bypass Condition 

1. Disconnect the brake tubes from the master cylinder.
2. Plug the outlet ports of the master cylinder.
3. Apply the brakes. If brake pedal height cannot be maintained, the brake master cylinder has an internal leak and a new brake master cylinder must be installed.

Brake Master Cylinder - Compensator Port 

The purpose of the compensator ports in the brake master cylinder is to:

• supply additional brake fluid from the brake master cylinder reservoir needed by the brake system due to brake lining wear.
• allow brake fluid to return to the brake master cylinder reservoir when the brakes are released. The returning brake fluid creates a slight turbulence in the brake master cylinder reservoir. This is a normal condition and indicates that the compensator ports are not clogged.

Clogged compensator ports may cause the brakes to hang up or not fully release. If clogged compensator ports are suspected, proceed as follows:

1. With the vehicle in NEUTRAL, position it on a hoist. Refer to JACKING AND LIFTING article.
2. With the brakes released, attempt to rotate each wheel and check for any brake drag.
   • If an excessive amount of brake drag exists at all 4 wheels, continue with the test.
   • If an excessive amount of brake drag exists at only one wheel, it indicates a possible seized brake caliper, brake wheel cylinder or parking brake component. Repair or install new components as necessary.
3. Check the brake stoplamp switch and the brake pedal free play to verify that the brake pedal is not partially applied.
4. Loosen the brake master cylinder nuts and position the brake master cylinder away from the brake booster.
5. With the brakes released, attempt to rotate each wheel and check for any brake drag.
   • If the brake drag is no longer present, install a new brake booster. Refer to POWER BRAKE ACTUATION article.
   • If the brake drag is still present, install a new master cylinder. Refer to HYDRAULIC BRAKE ACTUATION article.