JSchmitz,
Thank you, I'm well aligned with your description and appreciate another perspective.
We all understand it's differential pressure that moves fluid.
The differential in my car's fuel system is created by the pump diaphragm. The atmospheric pressure acting on the fluid in the tank does not change. It is static and therefore not the source of the differential.
The pressure inside the pump has to be lower than atmospheric pressure in the tank or the the fluid would not move from tank to pump. I call this suction. Others may not.
And I understand your comment about "a slight measurable pressure less than atmospheric". The differential is small because there is little resistance to flow. The gas flows quickly to eliminate the pressure differential created by the moving diaphragm... so fast I'm not really sure it could be measured, at least not with a mechanical vacuum gauge.
The only other way to create differential pressure between tank and pump is to pressurize the tank. This is not happening with a mechanical pump and a vented tank.
This is not worth a water fight post squabble in any way and I appreciate everyone's contributions to "how to prime a mechanical pump". I personally got the answers I needed and regret asking about suction.
My pump is primed and I'm happy about it.
Mike
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