I believe there is a check valve -- but not positive. I'd be looking for a leak in the fuel supply line, somewhere between the tank and the filter, or even between the filter and the pump. Believe it or not, sometimes the hard lines wear through in contact with other surfaces. I'd also check the O-ring on the drain petcock. When changing a fuel filter, I opened mine to drain it. On closing it, I crossthreaded it, and it was sucking in air. Started fine, but died shortly.
You don't need a lift pump if the Injection Pump is healthy. Some owners have found that adding an inline pump does help, because their Injection Pumps are weak.
Jack
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'83 Isuzu P'up Diesel LS - Original Owner
JoeIsuzu@IsuzuPup.comIsuzuPup.com