Hence, my personal theory is that the pumps probably run dry with less than 453 kg on ground in certain situations when the aircraft is parked on a downslope or maybe even just when empty and the gear struts extended.
I can see what you're saying there cosmo but then wouldn't it be a massive fluke that they just happen to run dry at 453 kg which is pretty much 1000lbs? Why wasn't it 353kg or 553kg? Is it a fluke that 453kg is also the firgure boeing used for max imbalance ?To me, It feels like a handy round number the've used rather than when pumps actually run dry.