Functional Test

With the engine stopped, press the brake pedal several times to deplete the vacuum reservoir, then press the pedal hard and hold it for 15 seconds. If the pedal sinks, either the master cylinder is bypassing internally, or the brake system (master cylinder, lines, modulator, proportioning valve, calipers or wheel cylinders) are leaking.
Start the engine with the pedal pressed. If the pedal sinks slightly, the vacuum booster is operating normally. If the pedal height does not vary, the booster or check valve is faulty.
With the engine running, press the brake pedal lightly. If the pedal sinks more than 10 mm (3/8 in.) in three minuets, the master cylinder is faulty. A slight change in pedal height when the A/C compressor cycles on and off is normal. (The A/C compressor load changes the vacuum available to the booster.