There is nothing wrong. The booster diaphragm is evacuated on both sides by the vacuum from engine manifold. When you apply brakes, a valve is shut to back of diaphragm and the vacuum is bled off the back side of diaphragm so the vaccum pulls (or more correctly the atmospheric pressure on back side pushes) the diaphragm forward to help you apply pushrod into back of master cylinder. When you pump brakes the vacumm has to be replenished rapidly and the volume is enough that it constitutes a large vac leak on a 4 cyl engine.

If you do this on a big cu inch V8 the volume is much smaller compared to engine air needs at idle so the flow from this makes little difference.

The booster test is to turn off engine and push pedal several times to bleed out all vacuum. Start engine with foot on pedal and see this it drops an inch +/-.