When you push the black primer bulb do you see any other of the clear lines with air bubbles? Look carefully at the line that goes into the IP (it's almost directly across from where your finger is, is comes from below the fuel filter bowl), the line that
goes into the black primer bulb and then the clear primary fuel filter. Old hose can have a bad habit of not leaking fuel but will suck air in. When I first got my car I decided to use a diesel purge type product and boy did it clean out the system; it also showed me where to look for leaks as it seemed to have loosened all the years of old crud in the lines and they started to leak.