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At Viking's recommendation, we installed the restrictor orifice between our header tank and the outlet side of the low pressure fuel pumps.
Background. This orifice is to prevent over pressure of the low pressure fuel system, by chance the high pressure fuel pump leaks back after shutdown.
One modification we made to the orifice installation, is that we also installed a check valve between the restrictor orifice and header tank. This check valve allows us to perform maintenance on our fuel system without concern that fuel would continue to leak through the orifice after opening up the system. The check valve we installed is the same one currently used to protect the low pressure fuel pumps, and threads onto the restrictor orifice.
Yesterday, I needed to replace a fitting at our fuel flow sender, located between our low and high pressure fuel pumps. Because of the check valve, I did not have to isolate or drain fuel between the orifice and wing tanks.