You're sucking air into the fuel line somewhere or else you have a fuel line collapsing. I don't think it's the latter because the fuel lines on a diesel should be metal, not rubber, but you need to verify that.
Might be a bad injector pump. I don't know how to check for that.
I would check the connections. If there is no apparent leak, there could be a crack in the fuel line at the top of the tank. That would explain why it could pull air.
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