Which tests must a vacuum brake booster pass during diagnosis?

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Multiple Choice

Which tests must a vacuum brake booster pass during diagnosis?

A vacuum brake booster is tested on three fronts: it must function properly, it must not leak when the brakes aren’t being applied, and it must not lose vacuum while the brakes are being applied. For function, you check that when you apply the brake with the engine running, the pedal becomes easier to press and the braking feel is нормal, indicating the booster is providing assist. If the pedal stays hard or the braking feels weak, the booster isn’t functioning correctly. For unapplied leakage, you inspect the booster with no brake application. After drawing a vacuum, the pedal should remain steady when you hold it, showing there’s no air leaking into the booster or past the check valve. If the pedal sinks or you hear a hiss, that signals a leak. For applied leakage, you monitor the system while the brakes are being applied. The pedal should not gradually lose vacuum or travel excessively under braking, which would indicate vacuum leakage during braking. Since a good booster must pass all three checks—function, unapplied leakage, and applied leakage—the most complete evaluation is that it must pass all of the tests.

A vacuum brake booster is tested on three fronts: it must function properly, it must not leak when the brakes aren’t being applied, and it must not lose vacuum while the brakes are being applied.

For function, you check that when you apply the brake with the engine running, the pedal becomes easier to press and the braking feel is нормal, indicating the booster is providing assist. If the pedal stays hard or the braking feels weak, the booster isn’t functioning correctly.

For unapplied leakage, you inspect the booster with no brake application. After drawing a vacuum, the pedal should remain steady when you hold it, showing there’s no air leaking into the booster or past the check valve. If the pedal sinks or you hear a hiss, that signals a leak.

For applied leakage, you monitor the system while the brakes are being applied. The pedal should not gradually lose vacuum or travel excessively under braking, which would indicate vacuum leakage during braking.

Since a good booster must pass all three checks—function, unapplied leakage, and applied leakage—the most complete evaluation is that it must pass all of the tests.

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