It's been my experience that something that leaks water under pressure leaks more air under vacuum.
If there is no check valve at the pump, and the static water level in the well is say .. 30' below the level of your tank, then the water in the pump pipe will try to draw down to the static water level in the well casing, it will try to draw a vacuum behind the check valve at the tank. Any tiny leak exposed to air between the static level in the well, and the check valve at the tank will let air get pulled in.
With a check valve at the pump, the water in the pump line cannot draw down and creates little or no vacuum between pump and tank, so no air gets pulled in.
Shallower wells, or wells with a high static water level (compared to tank) can pull this vacuum without problems even without a pump check valve (as long as they are well sealed). As a well gets deeper, and the difference between static well level and tank increases the amount of vacuum increases dramatically, requiring a check valve at the pump.
It kind boils down to this ... for air to get in your system there has to be a hole somewhere exposed to air ... so that narrows down your search to things that aren't under water
Seems obvious that you know this stuff already.
My pump is down about 125 feet on crappy old pvc pipe, with a good hand hold (T-pipe on pitless adapter) I can lift the pump, didn't say I could pull the pump, but I can lift the whole thing.