F 150 4WD V8-5.4L Flex Fuel (2008)
Vacuum Brake Booster Check Valve: Testing and Inspection
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.
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If manifold vacuum is available, stop the engine, connect the vacuum hose to the check valve and continue this test.
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If manifold vacuum is not available, stop the engine, connect the vacuum hose to the check valve and 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.
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If power assist is present, continue with this test.
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If power assist is not present, refer to Brake Booster Component Test. See: Testing and Inspection/Component Tests and General
Diagnostics/Brake Booster
6. NOTE: Do not remove the brake booster check valve from the brake booster in this step.
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.
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If there is enough vacuum for at least one power assisted brake application, the check valve is functioning correctly.
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If there is not enough vacuum for at least one power-assisted brake application, continue with this test.
8. Inspect the brake booster for any signs of damage that could cause a leak.
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If any damage is found, install a new brake booster and repeat this test.
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If no damage is found, install a new check valve.
