There is nothing such as "maximum vacuum pressure". Pressure and vacuum are opposite; we say "pressure" to mean above atmospheric pressure, and "vacuum" to mean below atmospheric pressure. Also, the question doesn't make sense in context of what you are trying to accomplish. A running engine—Slant-6 or other—creates plenty of manifold vacuum at idle (around 20" at sea level) and cruise conditions, less vacuum under acceleration, and zero vacuum at WFO (wide-open throttle). A vacuum reservoir with check valve—like this, this, this, this is used to store vacuum so the HVAC modes can still be shifted even with the accelerator on the floor on the way up a mountain.