If you are getting air into the filter housing even if drawing directly from the 5 gallon tank then there must be a leak in the Racor
Did you install the o-rings between the top housing and the tops of the filters?
Do you see bubbles anywhere?
See any drips or leaks
What about the hoses? I have had a hose with an invisible pinhole leak that let air into the system.
If you can't track down the leak then try pumping the fuel into the filter with the outlet closed so they filters are under a slight pressure. Don't overdo it.