One way to find an air leak is to pressurize the whole system like this:
The Diesel Page, Electric Lift Pump and Racor Filter installation
Pump can be mounted right after the tank outlet for testing. I believe some have a "flow through" feature that allows the electric pump to be left in place and used only for bleeding or emergencies if the regular pump fails. If the regular pump's diaphram fails fuel could possibly leak through into the crankcase? If using the electric pump on an ongoing basis it might be a good idea to by-pass the diaphram pump?