AA:

1) What is the ambient temperature when trying to start?
2) In the event that you have not done this, use a piece of dowel or other reaching tool to depress the air flow sensor plate while cranking. This will require the cooperation of the aforementioned helper to operate the key switch, while you depress the sensor plate.

From your description above, it seems that there is pressure in the lower chambers of the FD, but not enough in the upper chambers.