How can I prime the pump in my Craftsman 16 gallon Wet/Dry Vac?

Hi everyone,
I have a Craftsman 16 gallon Wet/Dry Vacuum with Built-In Pump, Model 338.17923, and am no longer able to prime the pump that pumps out water from a separate outlet. I ensure the opening inlet deflector is facing the top of the tank, and the pick-up and lid cages are properly installed. I then use the red wet pick-up elbow as per instructions.
If anyone has any suggestions, I would be very greatful. Thanks for your time and attention.