Component Tests and General Diagnostics

COMPONENT TESTING

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, 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. See Service and Repair.

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), replace the check valve and retest. If the brake pedal feels spongy, bleed the hydraulic system to remove air. See Brake System Bleeding.

Check Valve

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

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 diagnose the no vacuum condition.
1 Connect the vacuum hose to the check valve and run the engine for at least 10 seconds.
2 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.
1 Stop the engine and disconnect the vacuum booster hose from the brake booster check valve.
2 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-assisted brake application, the check valve is functioning correctly.
- If there is not enough vacuum for at least one power-assisted brake application, continue with this test.
1 Inspect the brake booster for any signs of damage that may cause a leak.

- If any damage is found, install a new brake booster and repeat this test. See Service and Repair.
- If no damage is found, install a new check valve.

Brake Master Cylinder - Bypass Condition Test
1 Disconnect the brake lines at the brake master cylinder.
2 Plug the outlet ports of the brake master cylinder.
3 Apply the brakes. If brake pedal height cannot be maintained, the brake master cylinder has an internal leak and must be rebuilt or replaced.

Compensator Port Check

The purpose of the compensator ports in the brake master cylinder is to supply any additional brake fluid required by the system due to brake pad wear and to allow brake fluid returning from the brake lines to the brake master cylinder to enter the brake master cylinder reservoir.
The returning brake fluid will cause a slight turbulence in the brake master cylinder reservoir. Turbulence seen in the brake master cylinder reservoir upon release of the brake pedal is normal and shows that the compensating ports are not plugged.
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. See Lifting.
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.
1 Check the brake stoplamp switch and the brake pedal free play to verify that the brake pedal is not partially applied.
2 Loosen the brake master cylinder nuts and position the brake master cylinder away from the brake booster.
3 With the brakes released, attempt to rotate each wheel and check for any brake drag.

- If brake drag is no longer present, install a new brake booster. See Service and Repair.
- If brake drag is still present, install a new master cylinder. See Removal and Replacement.