Ford maverick problems?

i've just bought a ford maverick with what i think is a fuel feed problem. when you prime the lift pump the car runs ok for a short while then loses power until you prime the lift pump again. the bloke i bought it off said he placed the engine with a known good one and a good diesel pump. i've put a one way valve on the fuel return hoping that would make the lift pump stiff but it doesn't work. i was told that the lift pump plunger will go solid when it's pumped up properly and stay like it but you could pump it all day and it won't go stiff. could the cause of my problem be the lift pump and air is getting into the fuel via a faulty diaphram. or could it be something else. thx