You could try cracking the pressure line on the output side of the pump, that should let the fluid start flowing but this method is very messy.
One method i use is to take the suction line from the tank to the pump loose at the pump, get some hydraulic oil in a container, use a funnel and fill that line with oil. Then run the pump in brief bursts till it picks up the fluid and starts flowing.

Whats going on here is a hydraulic lock effect caused by the air in the line.