95 7.3 powerstroke hrd start

I have a hard start condition on a 95 f250 with the 7.3 powerstroke. It starts until the engine oil pressure goes up while cranking. I thought the high pressure oil pump was bad, so a replaced it with a known good one. But it still does the same thing. Glow plug system is working fine. Have rpm signal. The high pressure oill reservoir is always full. The engine cranks and doesnt start until you see the oil pressure gauge go up. Any Ideas???

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i recently fixed one, it was the plastic fuel drain valve on the backside of the fuel filter, it was not leaking fuel out but sucking air into the system. replaced it, fired right up