how much air does it leak ? and thru what sort of hole ?
if its a factory intended leak, i would expect it to be thru a simple round hole in the case
power brakes are supposed to retain some vacuum if the engine dies - its a "save-your-ash-from-dieing" feature so that if the engine cuts out, you can bring the car to a safe stop
find a good size parking lot, or even an empty street at night and try simply shutting off your engine as you drive 40, wait about 2 seconds between hitting the switch and hitting the brakes, you should still have the power to safely stop the car.
if you car fails this test, plug the hole, try again, see if it changes