Component inspection and some general brake system checks can help isolate problems.
Check the brake booster first by pumping the brake pedal approximately 10 times (vacuum-assist systems) or 20 times (hydraulic-assist systems) with the engine off. While holding the pedal down start the engine. The pedal should fall slightly. If not, check for any visible faults before suspecting a faulty brake booster.
Check for leaks in the power booster vacuum line. A faulty check valve or a faulty O-ring between the master cylinder and the vacuum booster will decrease the vacuum assist. Low engine vacuum could also decrease vacuum assist. Test the check valve by removing it from the booster vacuum line and blowing through it. Air should pass through in the direction of the arrow, but not pass through the opposite way.
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