I think the design paremeter by most manufactures is enough vaccum retention in the booster for at least one power brake application if engine say quits on the road. If my thinking is rational and system in good leakproof condition that retained vaccum should be there when you first start your car. Then required vaccum operational build time is also reduced signifigantly if so. As the vaccum pump becomes perhaps older and less efficient everything would still appear normal. You would just be topping off the system at worse. Unfortunatly in the real world there are variences to put it mildly. Or another way to look at it. Mercedes had no optional charge for all the potentially active but unplanned future bleed off points in their choice of vaccum circuit components and materials.