[bestgenman]

I’ve changed filters on dozens of 8V Detroits during my business days servicing generator sets.  It has always been a practice to never remove both filters at the same time.  Doing so allows air to enter as the fuel runs backwards out of the lift pump.  The lift pump is after the primary and before the secondary and Detroit gear pumps do not do well with self priming.  

Occasionally, air would enter and the fix was to install the new primary (being the first filter in the flow of fuel), and use any device that would pull a vacuum at the secondary.  Sometimes it is necessary to remove the inlet line to the secondary as a good access point. Once the lift pump has fuel all is usually well.