[77newell]

A quick test would be to disconnect at the check valve on the inlet of the primary filter the fuel line and then hook a temporary line into a bucket of fuel, fill the filters and see how the engine runs. If it runs fine the problem is somewhere upstream of the check valve and if it still fails the problem is downstream of the check valve. The DD shop near Washington DC did this and eventually found that the pressure relief valve in the fuel pump was stuck open.

The fuel pump is a gear pump and you will not be able to blow fuel through it, I suspect the air you are getting is coming backwards through the fuel return line to the tank when you pressurized the tank.

Since you didn't mention changing anything between the time before the problem existed and now when it does I do not think the check valve being installed backwards it at all likely. Either air is getting to the system through a small leak or more likely if even when the tank is pressurized the engine fails is that the interior of the fuel hose is collapsed somewhere or the hose has become kinked somewhere prior to the check valve since both filters empty when the engine is running.

Jon
77 coach