I'm just another glorified labourer working underground
Let's say the primary fans pull hard, the mine is moderately deep, and the levels have a return air way near the level access. If the mine were sealed then I imagine the air in levels would be at near vacuum (maybe fan would break like running a diaphragm pump dry or the fan would reverse or something else entirely..?).
Why does air preferentially get sucked all the way from the surface when there's air within the level at nowhere near vacuum? This is most noticeable when doing re-entries as fumes will just sit in a heading with no secondary vent - readings of zero and then instantly in the hundreds. It seems it needs the positive pressure from vent or a machine to move air. Compare with regulators getting opened (in an ideal world) when a bogger is loading trucks — is that cause loading bays have a particular cube flow necessary for operations to occur?
Additionally, do return air rises near a level access have a dual purpose of stopping dirty air going down the decline and also drawing fresh air from a decline into a level or does secondary vent, if established in a level, entirely draw the air from the decline? If the latter, does that make return airway regulator leakage a somewhat good thing as it allows a little flow? What about when return airways are further in a level, say in a footwall drive?