Why do I need to check the oil level when the pump is running?

The reason for this is that if the vacuum is not broken before pumps are shut down the oil in the cover will seek the vacuum still in the cartridge and intake chamber. Then the oil level will drop in the sight glass and give the appearance of a low oil level. Then if the pump is refilled to the oil level line and the pump started, the oil that got sucked back into the cartridge and intake chamber will be kicked back into the cover and now you’ll be over filled and the oil will shoot out the handle (exhaust port).