Assuming that there is no problem with the engine creating vacuum, there is either a vacuum leak in the vacuum hose to the power brake booster unit, the check valve between the power brake booster unit and the vacuum hose is leaking, or the brake power booster unit needs to be replaced.
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Assuming that there is no problem with the engine creating vacuum, there is either a vacuum leak in the vacuum hose to the power brake booster unit, the check valve between the power brake booster unit and the vacuum hose is leaking, or the brake power booster unit needs to be replaced.