A crankcase should always have negative pressure in it, by which I mean lower than atmospheric pressure. That is what the pcv system is for. On a normal vacuum gauge you will not see a vacuum until you raise the engine speed and then not much. Crankcase pressure is typically measured with a magnehelic gauge because is it so low. If the crankcase has pressure in it the pressure will attempt to force oil out past any gasket or seal that presents the path of least resistance. Pushing oil past the intake valve seals is aided by the vacuum in the intake port because the pressure drop across the seal there is the greatest.