bosch pump hissing air.

[fieldofrob]

hi folks, my Bosch IP is still leaking air, not a drop of diesel comes out, clean as a whistle on the outside. but its letting air in somewhere, fuel line has lots of air in, all lines changed tho and filter etc leak off line's all done.
I cranked the injectors and lines and fuel return banjo up, but I can still hear air when I prime with pump. I'm stumped. sounds like its coming from near the top of the pump, like near the boost compensator.... anyone got any ideas? thanks.