The cab filter allows clean air to the positive (atmosphere) side of the diaphragm on the booster unit. The only time you should hear air is when you release the brake pedal and the internal spring pushes the diaphragm back to it's resting position forcing the atmosphere air back out. If you hear air being sucked in, you have a leak in the diaphragm. The leak could be small enough that the booster still works, just not as good as it should.
Mike B
